At the moment, due to budget problems and lack-of-choice at my local site, I just use white everything.
In the past I used to use a black/white (or green/white) mix to simulate a tracer; one white to every five-ten black (dependent on the mix, I used to keep varied jars for different situations), I found this far more effective than using an actual tracer, for a start, opponents are less likely to clock it, an actual tracer round is an active light source and draws the eye instantly, normal white pellets just reflect light which doesn't draw nearly as much attention (one of my concerns was people covering whoever I was shooting, I was more concerned about them clocking my shot than the person who's just been knocked out the game by it).
My mix ratios would tend to be decided by the environment I was playing. 1:5 would be for CQB because I'd need more tracer rounds to make sure I was on target quicker, especially with semi auto being my main fire mode. 1:10 was for support gunning and I usually reserved a mix of about 1:8 for woodland and outdoor. Obviously they didn't always come out the mag in those exact ratios, but they did average out alright.
If you're going to mix two types of ammo to create a basic tracer effect though, make sure your ammo is of equal quality and both types fire equally out of the gun they're being used in.
I've used proper tracers in the past as well but never liked the idea of sticking them through a silencer, I decided to build a tracer unit into my L85 to fix this.
I've just ordered two cheap gas grenades and I'll be putting whatever .12g I can find cheapest in them, preferably yellow.
Hope that helps.