win = CreateWindow("hallo",100,100,300,200,Desktop())
img = LoadImage("c:\post\test.png")

skin_window(win,img,$FF,0,0)

Repeat
   If KeyHit(1) Then Exit
Forever

Function skin_window(window, img, r, g, b)

   w = ImageWidth(img)
   h = ImageHeight(img)
   LockBuffer ImageBuffer(img)
   mask = (255 Shl 24) + (r Shl 16) + (g Shl 8) + b

   hWnd = QueryObject(window, 1)
   hRgn = api_CreateRectRgn(0, 0, w, h)
   For y = 0 To h - 1
      For x = 0 To w - 1
         pixel = ReadPixelFast(x, y, ImageBuffer(img))
         If pixel = mask
            hTempRgn = api_CreateRectRgn(x, y, x + 1, y + 1)
            api_CombineRgn(hRgn, hRgn, hTempRgn, 3)
            api_DeleteObject(hTempRgn)
         EndIf
      Next
   Next
   UnlockBuffer ImageBuffer(img)
   
   api_SetWindowRgn(hWnd, hRgn, True)
   api_DeleteObject(hRgn)

End Function