Moving Background Starfield

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Third time of writing, having some difficulty in explaining what I am trying to do.

Imagine the old game defender,
Put some stars in the background,
Add some paralax,

Easy peasy in 2d.

I want to get that effect behind a 3d scene. But do it before renderworld() and it gets cleared, do it after and the stars are "in front" of the 3d scene.

Been messing around with readpixelfast and writepixelfast but don't seen to get the right results with this,
	For back.stars = Each stars
	
		back\x = back\x + back\z
		If back\x > 800 Then back\x = back\x - 800
		If ReadPixelFast (back\x,back\z,BackBuffer())=-16777216 Then
			WritePixelFast (back\x,back\y,-1,BackBuffer())
		EndIf
		
	Next


It works, but does not seem to detect "not black" pixels to not write over. What argb value should I be looking for. Or is this wrong wrong wrong....