They seem to be tracer bullets, often used for instance to correct the aim while shooting. No relation with laser, of course, just a pyrotechnic effect.
As for the colours, a commenter on a gamers' forum (whose reliability I don't know about, but whose remarks seem sensible) observes:
The tracers actually looked pretty realistic for the types of weapons they were firing. Red and green are also accurate colors as were the amounts. Keep in mind countries use whatever metals are most abundant within the area to use for tracers. Germany happens to be rich in phosphorus which burns usually a bright green or bluish-green. The US/Canada happen to have a lot of strontium and sometimes will mix in a bit of magnesium to give it a bright red glow. Another reason there were a lot more tracers is most machine guns on tanks did not have the typical 4 ball ammunition to 1 tracer round, it was usually a 3:1 ratio due to gunners having to typically "free-aim" the MG instead of having a tradition traverse and elevation mechanism for better aim.
As for the abundance and outlook of the effect, it might have been tweaked a little for style purposes. A website about guns remarks:
Also, in the scene where the tank platoon rescued the pinned down infantry in the field, the tracers looked more like star wars laser beams than normal rounds. They first engaged the enemy at what had to be 600 yrds or more, but none of the rounds arched at all and they seemed to be moving at a higher velocity than what they should.