Color replace

Hi there - sorry to post query here - I need some feedback please...
I'm fairly new to GD and I want to composite a few 2color gif imgs on top of each other to create 1 final img, i am feeding my code with 2 color vars which will set the black on each gif color layer and leave white transparent. If i set colorAllocate to the var will it change the black to desired? I then merge those colored layers (with white transparent) and output final img...
Any help would be much appreciated!
Josh ;)

You can declare any color transparent by calling ImageColorTransparent (instead of ImageColorAllocate), or use ImageColorAllocateAlpha to use alpha channel.

Thanks I'll attack that now - the gif img layers are black and white - I want to set the black to the given var hex2rgb array and the imagecolortransparent white:

How to replace the black with hex(?)

$black = imagecolorallocate($im, hex2rgb[r],hex2rgb[g],hex2rgb[b]);

$white = imageColorAllocate ($im, 255, 255, 255);
$trans = imagecolortransparent($im,$white);

Thanks heaps?! ;)