You disable most, if not all, of the additional custom reticle features in the Add-On's Settings menu for
Rainbow Reticle by
Mitazaki.
It doesn't have a particular distance based coloration, so once you are within range weapon distance of an enemy, it will show red, no matter if you still too far for a melee hit or not.
I don't think it would possible to make add-on that would be 100% accurate about being close enough for successful melee strikes, but perhaps say if you are within a fixed distance, say 1 metre of the enemy (or whatever is the appropriate distance) might be determinable.
Hope that helps.