I used to play all my colonial games with either The Sword and the Flame (the quintessential colonial ruleset) or Fields of Glory. TSATF was for larger scale figures and Fields of Glory was geared more towards 15mm or smaller sized forces. Unfortunately I don't think either are in print anymore, though you might be able to get a hold of them on ebay.
Ultimately I moved from straight colonial battles into Darkest Africa gaming instead. After a while the thin red line shooting down hordes of tribal/lesser equipped natives got really boring for me. Darkest Africa focuses on the colonization of interior Africa in the 19th century and is more of a skirmish scale type game with a few white explorers leading small contingents of native levies, askaris and colonial troops to conquer places that people really didn't need to bother with. The forces range from the colonial powers (British, Belgian, German, Porteugese and French) to tribal villagers (Azande, Masai and the like) to arab Zanzibari slavers and their Ruga-Ruga mercenaries. Lots of variety involved. Rules I used were In the Heart of Africa, by Chris Peers.
I have found discussions on Chris Peers updating his In the Heart of Africa rules in a volume called Death in the Dark Continent. Some have said these rules are a not too bad, though less detail focused replacement for the Sword and the Flame, though
YMMV.
Unfortunately one of the greatest colonial wars sites ever on the web has finally disappeared: General Tremorden Reddering's Colonial wars page. A great page that used to detail the fictional colonial battles for the land of Ouragistan and included everything from rules and mini-manufacturers links to plans for making colonial terrain pieces and massive steamed power landships for colonial gaming. So sad to see that it is finally gone...:(
Skriker