Colours and sizes often aren't very helpful in identifying insects. It is the shape of the things that is diagnostic, and if you were capable of describing that in words that enabled us to identify it, you'd probably already have the knowledge to work out the answer yourself.

If they are literally "flies", ie diptera (members of the family containing houseflies, bluebottles, hoverflies, mosquitoes, gnats), then most of them are harmless and some are beneficial.

On the whole, if they don't look like aphids, I'd probably leave them alone.

Just because they don't look big enough to eat something else, doesn't mean they don't. The parasitic wasps which "eat" various nasty things are in some cases so small you don't even see them. They do the eating by laying their eggs inside them, and then their larvae eat them from the inside.

We won't have much clue unless you can get your macro lens out and take a piccy.