The easiest way to do this would be just going through the game settings, but most oldies are too stubborn to use it. Something like this should work otherwise
model bitterman
cg_forceTeamModel bitterman
cg_forceEnemyModel keel/bright
/model ranger/sport_red
Skins are red blue sport sport_red sport_blue
There's also a /skin cvar (/skin sport), no idea which method is working, check yourself