I tried to fix it by disabling a bunch of unneeded scripts (it didn't work, but it didn't hurt anything either, so you could keep it like that or put the scripts back, it doesn't matter. The game still behaves the same as far as I know.) Also, the only way I was able to solve the problem was by deleting the light blue costume. If you still want light blue as an option, try adding it again, but switching its order in the list of costumes or something. But, anyway, it appears to be working now!
Well, actually, never mind about it working fine. I found another glitch :(, which I will try to fix. ^^Double nevermind XD. The glitch was just that if you don't click the slime (because you want it to be black) it won't show up. I solved the problem just by cycling through the colors until I got to black again. So, it should work now. ^Ugh. Triple nevermind. Some colors work, but others get replaced by the dark blue :(. Well, maybe you can find what's wrong....or delete all the blue costumes: they appear to be slightly evil XD