Laser Beams

why are the shots fired from ground units green,and so on?

Tracer rounds.

For a very long time I thought that tracers were only red. Was wrong, turns out tracer bullets often feature green:

"In NATO standard ammunition (including US), the oxidizer salt is usually a mixture of strontium compounds (nitrate, peroxide, etc.) and the metallic fuel is magnesium. Burning strontium yields a bright red light. Russian and Chinese tracer ammunition generates green light using barium salts."

https://en.wikipedia.org/wiki/Tracer_ammunition

There are also dim tracers and invisible to the eye tracers that emit an IR light, so that only troops using night vision goggles/sights can see the trace. Not in WW2 mind you, but anyway.

There may be other colors, but I only know of red green and IR for certain. I think there may be a yellow.

Many pilot's histories describe being shot at by big bright green golfballs, over Vietnam, among others.

For german, you could find red, yellow, blue and white tracers if I remember correctly.

why

why did they use them

To see where you are shooting at

Tracers also have some psychological sideeffect.

why are the shots fired from ground units green,and so on?

Here are some red and yellow tracers - you can see that in real life they look like lasers too.

4:42 is a good place to watch a nice line of tracers.

This video is of some single-round green tracers:

why did they use them

They are still in use by pretty much all militaries around the world. They're mostly used for machine guns and help the gunner to adjust his him as he can see where his shots are flying to.