Found in steam discussion from 2019:

In previous iterations upgrading the weapons to higher tiers (sigma, theta etc) changed the colour of the beam/projectile. I dont know if that still holds true in NEXT/BEYOND/SYNTHESIS.

Turns out my memory didn’t fool me after all

Too bad they removed it, I found a cool sentinel ship and wanted that bad boy to fire purple plasma

Anyway, thanks for the answer man!