diff --git a/src/buttons_effects.c b/src/buttons_effects.c
index 1ea9ec38..dcb336e9 100644
--- a/src/buttons_effects.c
+++ b/src/buttons_effects.c
@@ -180,6 +180,7 @@ static int Check_block_constraints(enum IMAGE_MODES mode)
   int x, y, x2, y2;
   int block_width = 8, block_height = 8, max_colors = 2;  // default values
   int error_count = 0;
+  byte errcol = 17;
 
   switch (mode)
   {
@@ -193,6 +194,10 @@ static int Check_block_constraints(enum IMAGE_MODES mode)
       block_width = 4;
       max_colors = 3; // 3 + background color
       break;
+    case IMAGE_MODE_GBC:
+      max_colors = 4;
+      errcol = 33;
+      break;
     default:
       return -1;  // unsupported mode
   }
@@ -210,11 +215,24 @@ static int Check_block_constraints(enum IMAGE_MODES mode)
         {
           int i;
           byte col = Main.backups->Pages->Image[0].Pixels[x+x2 + (y+y2)*Main.image_width];
+          if (mode == IMAGE_MODE_GBC)
+          {
+            if (count == 0)
+              c[count++] = col;
+            else if ((col & ~3) != (c[0] & ~3))  // compare palettes
+            {
+              if (Main.backups->Pages->Nb_layers < 2)
+                Add_layer(Main.backups, 1);
+              Main.backups->Pages->Image[1].Pixels[x+x2 + (y+y2)*Main.image_width] = errcol;
+              error_count++;
+            }
+            continue;
+          }
           if (col > 15) // forbidden color !
           {
             if (Main.backups->Pages->Nb_layers < 2)
               Add_layer(Main.backups, 1);
-            Main.backups->Pages->Image[1].Pixels[x+x2 + (y+y2)*Main.image_width] = 17;
+            Main.backups->Pages->Image[1].Pixels[x+x2 + (y+y2)*Main.image_width] = errcol;
             error_count++;
             continue;
           }
@@ -239,7 +257,7 @@ static int Check_block_constraints(enum IMAGE_MODES mode)
                 GFX2_Log(GFX2_INFO, "Check_block_constraints() intensity error at (%d,%d) color=%d (other color=%d)\n", x+x2, y+y2, col, c[0]);
                 if (Main.backups->Pages->Nb_layers < 2)
                   Add_layer(Main.backups, 1);
-                Main.backups->Pages->Image[1].Pixels[x+x2 + (y+y2)*Main.image_width] = 17;
+                Main.backups->Pages->Image[1].Pixels[x+x2 + (y+y2)*Main.image_width] = errcol;
                 error_count++;
                 continue;
               }
@@ -250,7 +268,7 @@ static int Check_block_constraints(enum IMAGE_MODES mode)
               GFX2_Log(GFX2_INFO, "Check_block_constraints() constraint error at (%d,%d)\n", x+x2, y+y2);
               if (Main.backups->Pages->Nb_layers < 2)
                 Add_layer(Main.backups, 1);
-              Main.backups->Pages->Image[1].Pixels[x+x2 + (y+y2)*Main.image_width] = 17;
+              Main.backups->Pages->Image[1].Pixels[x+x2 + (y+y2)*Main.image_width] = errcol;
               error_count++;
             }
             else
@@ -341,7 +359,7 @@ void Button_Constraint_menu(void)
     int grid;
   } modes[] = {
     {IMAGE_MODE_ZX,      "ZX Spectrum",   "2 colors per 8x8 block", 1}, // 256x192
-    //{IMAGE_MODE_GBC,   "Game Boy Color"},
+    {IMAGE_MODE_GBC,     "Game Boy Color","4 colors per 8x8 block", 1}, // 160x144 to 256x256
     {IMAGE_MODE_THOMSON, "40col (MO/TO)", "2 colors per 8x1 block", 1}, // 320x200
     {IMAGE_MODE_EGX,     "EGX (CPC)",     "Alternate Mode0/Mode1 ", 0}, // 320x200
     {IMAGE_MODE_EGX2,    "EGX2 (CPC)",    "Alternate Mode1/Mode2 ", 0}, // 640x200
@@ -394,6 +412,8 @@ void Button_Constraint_menu(void)
     }
     else if (clicked_button == 3)
     {
+      if (Selected_Constraint_Mode == IMAGE_MODE_GBC)
+        set_palette = 1;  // activate palette back when switching from GBC
       Selected_Constraint_Mode = Window_attribute2;
       for (i = 0; i < sizeof(modes)/sizeof(modes[0]) ; i++)
         if (Selected_Constraint_Mode == modes[i].mode)
@@ -401,32 +421,24 @@ void Button_Constraint_menu(void)
           set_grid = modes[i].grid;
           Hide_cursor();
           Print_in_window(10, 21+18, modes[i].summary, MC_Dark, MC_Light);
-          Print_in_window(10+3, 87+3, set_grid?"X":" ", MC_Black, MC_Light);
           Display_cursor();
           break;
         }
+      if (Selected_Constraint_Mode == IMAGE_MODE_GBC)
+        set_palette = 0;
     }
-    else if (clicked_button == 4)
-    {
-      // palette
+    else if (clicked_button == 4) // palette
       set_palette = !set_palette;
+    else if (clicked_button == 5) // picture size
+      set_pic_size = !set_pic_size;
+    else if (clicked_button == 6) // enable grid
+      set_grid = !set_grid;
+
+    if (clicked_button > 0) // refresh buttons
+    {
       Hide_cursor();
       Print_in_window(10+3, 51+3, set_palette?"X":" ", MC_Black, MC_Light);
-      Display_cursor();
-    }
-    else if (clicked_button == 5)
-    {
-      // picture size
-      set_pic_size = !set_pic_size;
-      Hide_cursor();
       Print_in_window(10+3, 69+3, set_pic_size?"X":" ", MC_Black, MC_Light);
-      Display_cursor();
-    }
-    else if (clicked_button == 6)
-    {
-      // enable grid
-      set_grid = !set_grid;
-      Hide_cursor();
       Print_in_window(10+3, 87+3, set_grid?"X":" ", MC_Black, MC_Light);
       Display_cursor();
     }
@@ -447,6 +459,10 @@ void Button_Constraint_menu(void)
             Resize_image(256, 192);
             End_of_modification();
             break;
+          case IMAGE_MODE_GBC:
+            Resize_image(160, 144);
+            End_of_modification();
+            break;
           case IMAGE_MODE_MODE5:
             Resize_image(288, 256);
             End_of_modification();
@@ -479,6 +495,7 @@ void Button_Constraint_menu(void)
         switch (Selected_Constraint_Mode)
         {
           case IMAGE_MODE_ZX:
+          case IMAGE_MODE_GBC:
           case IMAGE_MODE_C64HIRES:
             Snap_width = 8;
             Snap_height = 8;
@@ -517,6 +534,13 @@ void Button_Constraint_menu(void)
             Fore_color = 7;
             Back_color = 0;
             break;
+          case IMAGE_MODE_GBC:  // 32 colors among 32768
+            memset(Main.palette + 32, 0, sizeof(T_Components) * (256 - 32));
+            Main.palette[33].R = 255; // for color clashes
+            First_color_in_palette = 0;
+            Fore_color = 3;
+            Back_color = 0;
+            break;
           case IMAGE_MODE_THOMSON:
             {
               static const T_MultipleChoice moto_choices[] = {
diff --git a/src/graph.c b/src/graph.c
index 4dd24326..41989b11 100644
--- a/src/graph.c
+++ b/src/graph.c
@@ -3386,6 +3386,31 @@ done:
   }
 }
 
+/// Paint a pixel with GBC constraints
+///
+/// Same 4 color palette for all pixels in a 8x8 block.
+static void Pixel_in_screen_gbc_with_opt_preview(word x,word y,byte color,int preview)
+{
+  word startx = x & ~7;
+  word starty = y & ~7;
+  word x2, y2;
+  byte palette;
+
+  // first set the pixel
+  Pixel_in_screen_layered_with_opt_preview(x,y,color,preview);
+  palette = color & ~3;
+  // force all pixels of the block to the same palette
+  for (y2 = 0; y2 < 8; y2++)
+  {
+    for (x2 = 0; x2 < 8; x2++)
+    {
+      byte col = Read_pixel_from_current_layer(startx+x2, starty+y2);
+      if ((col & ~3) != palette)
+        Pixel_in_screen_layered_with_opt_preview(startx+x2, starty+y2, palette | (col & 3), preview);
+    }
+  }
+}
+
 /// Paint a pixel with C64 MultiColor constraints
 ///
 /// Only 4 colors in a 4x8 block, including the background color
@@ -3558,7 +3583,6 @@ void Update_pixel_renderer(void)
   switch (Main.backups->Pages->Image_mode)
   {
   case IMAGE_MODE_C64FLI: //TODO
-  case IMAGE_MODE_GBC:  // TODO
   case IMAGE_MODE_ANIMATION:
     // direct
     Pixel_in_current_screen_with_opt_preview = Pixel_in_screen_direct_with_opt_preview;
@@ -3575,6 +3599,9 @@ void Update_pixel_renderer(void)
   case IMAGE_MODE_THOMSON:
     Pixel_in_current_screen_with_opt_preview = Pixel_in_screen_thomson_with_opt_preview;
     break;
+  case IMAGE_MODE_GBC:
+    Pixel_in_current_screen_with_opt_preview = Pixel_in_screen_gbc_with_opt_preview;
+    break;
   case IMAGE_MODE_C64HIRES:
   case IMAGE_MODE_ZX:
     Pixel_in_current_screen_with_opt_preview = Pixel_in_screen_zx_with_opt_preview;
diff --git a/src/helpfile.h b/src/helpfile.h
index 18b614d4..aea7154e 100644
--- a/src/helpfile.h
+++ b/src/helpfile.h
@@ -2,6 +2,7 @@
 */
 /*  Grafx2 - The Ultimate 256-color bitmap paint program
 
+    Copyright 2018 Thomas Bernard
     Copyright 2008 Yves Rizoud
     Copyright 2009 Franck Charlet
     Copyright 1996-2001 Sunset Design (Guillaume Dorme & Karl Maritaud)
@@ -1936,10 +1937,22 @@ static const T_Help_table helptable_effects[] =
   HELP_TEXT ("")
   HELP_TEXT ("")
   HELP_BOLD ("Block modes")
-  HELP_TEXT ("ZX Spectrum, Thomson MOTO, C64 multicolor,")
-  HELP_TEXT ("C64 HiRes are modes with block constraints.")
-  HELP_TEXT ("Only a limited different colors (generaly 2)")
-  HELP_TEXT ("are allowed in a pixel block.")
+  HELP_TEXT ("ZX Spectrum, Game Boy Color, Thomson MOTO,")
+  HELP_TEXT ("C64 MultiColor, C64 HiRes are modes with")
+  HELP_TEXT ("block constraints. Only a limited count of")
+  HELP_TEXT ("different colors (generaly 2) is allowed in")
+  HELP_TEXT ("a pixel block.")
+  HELP_TEXT ("")
+  HELP_BOLD ("Game Boy Color")
+  HELP_TEXT ("All pixels within a 8x8 block should be in")
+  HELP_TEXT ("same 4 colors palette. There are 8 palettes:")
+  HELP_TEXT ("colors #0-#3, #4-#7, ..., #28-#31.")
+  HELP_TEXT ("Advice: put common colors at the same")
+  HELP_TEXT ("        position within the palettes. eg if")
+  HELP_TEXT ("        you need a black background, set")
+  HELP_TEXT ("        colors #0, #4, #8, etc. to black.")
+  HELP_TEXT ("Color #0 of each palette is transparent for")
+  HELP_TEXT ("Game Boy sprites.")
   HELP_TEXT ("")
   HELP_TEXT ("")
   HELP_BOLD ("Amstrad CPC Mode5")