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First, it seems a little weird to reallocate $white to some other color; and especially weird that neither of them are actually white.
Secondly, if you deallocate a color, you remove it from the images color table, which means any pixels drawn in that color would now be displayed in whatever new color you allocate to that entry, which is going to produce weird results.
If you insist on using paletted images, you should chose all your colors, allocate them to different variables and then use them. Not reallocate or deallocate them.
But paletted images are just a pain in the butt and relics of a bygone era. You'd be much better off using TrueColor (24-bits per perl) images, then you can forget about allocating colors completely and just use the rgb value converted to a suitable integer. I use the following simple function for this:
See ColorRamp1785 for usage.
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In reply to Re: allocating/deallocating colors with GD module.