firstly excuse my english, please, i'm not a native speaker.
ok, the answer! in san andreas the color of a territory, beit green, purple or yellow, is lighter if resistance on that territory is light, or darker if there is heavy resistance. on easy territories gangs come in one wave if the area is small, and have nothing more than guns and bats, while darker ones are protected by more enemies, loaded with micro smg and automated rifles and such.
if you loose a fight but managed to defeat one or two waves of enemies, the next time you attack that territory it should be easier. but keep in mind that territories tend to retain the strenght they have when you conquer them, so if you go on a strong area try to take it in one run.