On the official forums, there's a list of what tracers gives what colours.

Took a stock design, duplicated it and put in one of each of the following color tracer - Red, orange, yellow, green, indigo and violet (no blue, because no blue). Then mounted them on a custom design in that order and put it into a mirrormatchup.

So to answer your question, I'm using six different flavors of tracer (six different flavors of otherwise identical railgun).

WHAT THE HECK OF KINDS OF TRACERS WERE YOU USING?