If you open up your reticle picture there should be the hit reticle you just needs to overlay that colour with the new colour you want it to be when you hit someone.
No, that file only stores the default picture of the reticle.
When you hit someone, the colour is changed with an overlay in the engine, the colour of which can be changed in the scripts. In client/gui/guis/gameHud.gui, look for a line that says:
hitFeedbackColor = "255 0 0 255";
If you don't know how this is interpreted, the format is "Red Green Blue Alpha" where Alpha is basically the opposite of transparency, and 255 is the maximum for all four values.