Well, you think you don't alter anything but the color, but a img file with any extension (blp, png, tga, jpg) has a total of FOUR channels (RGBA) and a limited, or set AMMOUNT of colours.
R = Red
G = Green
B = Blue
A = Alpha
I bet you know the first three, but the later, is not so known and often forgoten.
It is what blizzard uses to store what parts should become transperant, or even reflective.
Then, the ammount of colour in the file, helps saving space. If the file is to big then the original WoW had, it will reject it. and place Green.blp in place of all missing blp's.
So check the size of your file, after you edited it, with the original. Try seeing what is differant, and how you can make yours look asmuch alike the original, in terms of size, colour count, and alpha channel(s).
Mostly this will solve your problem
