You dont edit the tga to affect color, they are all 3 channels for a reason.
To swap out individual crosshairs you edit the appropriate weapon.cfg file for that weapon, ie shotgun would be data-1.1.0.pk3/models/weapons/shotgun/weapon.cfg
The color line is in RGBA, ie
1.0 0.0 0.0 1.0 is red with 100% opacity
0.0 1.0 0.0 1.0 is green with 100% opacity
0.0 0.0 1.0 1.0 is blue with 100% opacity
You can miix and match to get other colors too.