Total Members Voted: 16
It's possible to change its colour but "depending on what you're aiming at" isn't: you cannot do raycasts/line checks with a clientside mod, and a server mod would have ping delays and generally be unresponsive.
...you cannot do raycasts/line checks with a clientside mod...
So in other words, the answer to your question, Azhang, is no. This is not possible without engine changes, which have an extremely slim chance of happening.
is it possible to have a green cross hair when not aiming at anything but then the cross hair turns red when aiming at enemy?
It would be more like white if you aiming at a brick with no projectile events, yellowif aiming at a brick, green if aiming at a freindly player (TDMs mainly), and red if at an enamy (possibly ennykind of mini-game).