Except for the fact that real lasers don't look like that at all... if anything it's "Star Wars"-style lasers/"blasters" that look like wwII tracers.
And as has been mentioned in the wikipedia article, tracers are not made with "a bit of magnesium at the end" (that would give a white flame, and probably wouldn't have the right incendiary qualities). They're made a hollow base filled with a mix of some metal fuel (magnesium perclorate is mentioned, but I'm sure there are other possibilities) and different salts. This is the same technique that is used in fireworks. Depending on which salts you're using, you get different coloured flames. Strontium for red tracers, barium for green. I'm not sure what the yellow tracers used by the germans used, but I'd hazard a guess at sodium.
In real life, gunners would often take the time to mix tracers and common rounds in their ammo belts for their personal preferences, so there's no point in saying "one tracers in x rounds is the correct amount", because it varies with availability of tracers, and the mission for which the gun is intended. RO uses a figure of 1 in 5 rounds, which is quite okay in my opinion for infantry combat.