I won't screw up on a Sato...

[Mirinee]

The color of a mag you find in game varies depending on which version of PSO you have (PSO+ or regular). In PSO, you find gray or grayish colored mags, so to get a nice colored one, you need to make a dummy character and steal it. In PSO+ the mags are the color of player 1’s costume. If player 1 has costume 8 or 9, then you will find all random sorts of mags.

I have the EU Version of PSO (not plus), and I think that new MAGs I find in the Mines assume the colour of the (clothes of the) first character to equip it.

What I definitely know is that they can be colours other than shades of grey.

There are differences in the US and EU versions of PSO Episode III, so there could very well be differences in the versions of PSO Ep I&II (not plus).:S

BTW, I like your Sato, Flix0rz.

Edit:
Yes, Flix0rz (flickynick) is correct. For the 1st row, e.g. the HUmar one, it's always red, and the second row always orange.