Heavy photons are not coded for in SFC1 or 2. Whatever change you make in the shiplist apparently has the effect of changing the color. I'm surprised the game reads it as anything at all.
I'm sorry but i have to disagree. I mean i replaced other things from OP to 1 and worked .
plasma S texture was replaced by plasma X texture and R one with the Snare one. they work just fine.
And why should it be such a surprise? i converted them to 256 PCX files.
I tried to make them *.bmp and played, an orion fired a plasma at me and instead of the texture, it appeared a white pixelated (big pixels 32x32) thing.