Hey Voyager, I had the exact same problem with you once upon a time. It's not very obvious how to fix it, but I worked it out in the end. I wrote about it in the
Converting to and from GRP graphics tutorial, but that's real long and most of it won't be relevant to you. Here's the relevant part:
QUOTE(BSTRhino)
The wireframe HP colors are very tricky. If you look hard, you'll notice that each of the four colors at the special indexes #208-211 appear three times in other places on the color palette, most noticeably at 216, 217, 218 and 219. What you will find is, in a lot of graphics programs, if you choose color #208 on the palette and start drawing your wireframe with it, then use your Info palette or something similar, you'll find the actual color that gets drawn on the image is #135. This is because, in RetroGRP's palette, color #208 has the RGB values of 252, 252, 56. The exact same RGB values can be found at indexes 216 and 135. To deal with the situation of the same color appearing multiple times in the same color palette, most graphic design programs just take the first index that matches the color you're drawing with, which happens to be #135. In this palette, the color would be right, but the index would be wrong, and so you'd most probably find your wireframe would not change with your unit's health.
One way to fix this is to edit the color table and change the colors at #208-211 to unique colors, or the #216-219 colors if that is what you're using. To change a color in the color table in Photoshop, open the color table by going to Image > Mode > Color Table and double click on a color to change it. Remember, the color palette you use is not saved to the GRP file, so even if it looks funny in your editor, it will give you the correct color indexes and therefore the correct colors when you play StarCraft.
The main idea is Photoshop or other programs will paint the right colour but the wrong colour index, and you need to get the correct colour indexes, the colours aren't important because they're not saved into the GRP. So that's what's causing the problem. If you use that eyedropper tool you can see that when you draw with the colour at #208 you'll get #135 on the actual drawing. I solved this by changing the colour at #208 and the other wireframe indexes. I hope that works for you too.
Does that help you?