Show us what you're working on, (preferably with screenshots).
I just say that bullets would go out in alternate colours so the player could tell when collisions are going to be checked. This may reduce computing time and still feel precise enough for the player.
Can you expand on this because it all sounds a bit illogical to be honest? When you say "alternate colours" do you mean alternating the game frames the bullets move on (half on even frames, half on odds) or something else because it's not clear what changing the colour of a bullet would have on accurate collision detection? Do you foresee a frame of the game where some bullets are not being checked for collision detection at all?
I don't know maybe you can disguise a ray by drawing a line as a gradient from light (bullet end) to dark (tail end) giving it an appearance of motion so that your fake bullets seem to travel a shorter distance than they actually do each time you check for collisions.