Trouble with Pixmaps

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REDi

I dont understand how I would handle the output of ReadPixel, or construct input for Write pixel either

Strict
Graphics 800,600,0
Local Image:Timage = LoadImage ("Cursor.bmp")
DrawImage(Image, 0,0)
ColourSub(Image,0,0,128, 200,140,0)
DrawImage(Image, 64, 0)
Flip
Delay 5000

Function ColourSub(Image:TImage,OldR,OldG,OldB,NewR,NewG,NewB)
	Local Pix:TPixmap = LockImage(Image)
	Local x:Int, y:Int
	Local OldColour:Int = (255 Shl 24)|(OldR Shl 16)|(OldG Shl 8)|OldB 
	Local NewColour:Int = (255 Shl 24)|(NewR Shl 16)|(NewG Shl 8)|NewB 
	For x=0 Until PixmapWidth(Pix)
		For y=0 Until PixmapHeight(Pix)
			If ReadPixel(Pix,x,y) = OldColour
				WritePixel(Pix,x,y,NewColour)
			EndIf
		Next
	Next
	UnlockImage(Image)
End Function