CreateImage Transparency not working

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I already had the image creation function on my site (built for me by someone else) but I thought I would add more images and put them into their own folder, since I was adding approximately 750 images to the 50 or so that were already there.

 

I've uploaded the new images (which took forever, by the way, so I can't change their format now) and decided to test out the function.

 

It has five layers - background, color1, color2, color3 and an outline. If I place one of the new colors in, it works. But add another new color, and it covers up the previous new color. I've read a whole lot of confusing jargon about PNG transparency issues and alpha blending and such, but I'm not sure exactly what to do in my situation. I'm pretty good with PHP but I've done very little work with images.

 

The script runs fine, no errors get returned or anything. I just need to set white as the transparent color, but I can't run through ALL the colors I use in the images (each image, color1 color2 etc has a whole lot of different colors in it).

 

I've tried contacting the person who wrote the original function, as all their old images work fine. But I've contacted them for issues in the past and they have very rudely ignored me, so I don't fancy my chances this time. I've had help on these forums before so I've got my fingers crossed someone can help me with this.

 

Many thanks in advance!