Colorizing a specific area on the screen.

i think this is what your after...
this is a very unoptimised replace olour technique.
usage

i:tpixmap = loadpixmap("image.png")

' replace all exact white occurances with bright purple
p:tpixmap = replacecolour(i:tpixamp,$ffffff,$ff00ff)

' replace all nearly white occurances with bright purple
p:tpixmap = replacecolour(i:tpixamp,$ffffff,$ff00ff,10)

functions

Function replacecolour:TPixmap(p:TPixmap,old_rgb:Int,new_rgb:Int,threshold:Int)
	Local w:Int = PixmapWidth(p)
	Local h:Int = PixmapHeight(p)
	Local x:Int,y:Int
	Local b1:Byte Ptr
	Local b2:Byte Ptr
	Local n:TPixmap = CreatePixmap(w,h,pf_BGRA8888)
	
	Local r:Int = (old_rgb Shr 16) & 255
	Local g:Int = (old_rgb Shr 8) & 255
	Local b:Int = (old_rgb Shr 0) & 255
	
	Local p1:TPixmap = ConvertPixmap(p,PF_BGRA8888)
	
	' scan through image
	x  =0
	While x < w
		y = 0
		While y < h
			b1 = PixmapPixelPtr(p1,x,y)
			b2 = PixmapPixelPtr(n,x,y)
			
			If threshold_check(r,b1[2],threshold) And threshold_check(g,b1[1],threshold) And threshold_check(b,b1[0],threshold)
				b2[0] = (new_rgb Shr 0) & 255
				b2[1] = (new_rgb Shr 8) & 255
				b2[2] = (new_rgb Shr 16) & 255
				b2[3] = $ff
			Else
				b2[0] = b1[0]
				b2[1] = b1[1]
				b2[2] = b1[2]
				b2[3] = b1[3]
			EndIf
			
			y:+1
		Wend
		x:+1
	Wend
	
	Return n
End Function

Function threshold_check:Int(v1:Int,v2:Int,t:Int)
	If (v1 > v2-t) And (v1 < v2+t+1) Then Return True
	
	Return False
End Function