After editing some wireframes they are now not solid green when the unit is fully healed; instead they're multicoloured as if they're half dead. Why is this?

Which ones did you replace?

I don't know if this has anything to do with unmodded wireframes, where they could be any variety of shades, such as blue-black for Protoss; red, orange, brown, yellow, green or any mixture for Terran; or wacky shades for Zerg. You may want to look in to the unit maybe having over 100%/100% hp.

Nvm, I'm pretty sure I used the units palette instead of the wireframe one. If it still happens when I change it back I'll let you know.

Although when I think about it that can't be the problem. I edited zeratul's wireframe slightly and only the edits are red. Then when I copied his entire wireframe including the edit, and pasted it onto the firwbat wireframes, the firebats were the ones which had the messed up wireframe.
Unit HP is not the problem.

The colors for the wireframes appear in the index multiple times - I believe it's the last place the appear (220-227 ish?) that you need to use for the wireframe to display correctly. Most graphics programs default to the first index that the color appears when you select it.

http://maplantis.mikelat.com/index.php?mod=452
If you use the "Icon-Wire" palette there are no conflicting colors.