As far as I am aware, the color itself is not critical (I have used different colors successfully)... what DOES matter is which index the color is in.
To explain... Using PhotoShop, open some other squad's noseart that works (you will find these in your HTC/AcesHigh/Squads folder). See what the background color is. Go to the Image Menu, then Mode, then scroll down to Color Table. A grid of 256 colors will come up... this is your indexed color table. Now find which square holds the background color of the artwork (I believe it's the last square, but it may be the first. I can't verify this right now, but it's one of those). This is why the color is usually an awful pink or green... it stands out from the rest.
Once again, the color itself is not important, but it's index is.