IMO all africans are active provided good water quality is maintained (which i know you would be giving them
)...with the color part, it all depends on your personal choice. there are so many of them and with so much color that i really get confused sometimes and want to put all of them in a huge, huge tank...
but i'll try to run through some malawi species, since i don't know much about tangs...
you can easily keep a trio(1m/2f) of any of the aulonocara (peacock) species or smaller copadichromis species (c. azureus is my favorite). the males have amazing color, but females are drab...so it's your call.
there are a number of victorian haps that would be comfortable there (1m/2f)...your call again, numerous species...
other non-mbuna like placidochromis, sciaenochromis, protomelas, nimbochromis etc are out of question...(well, not completely, but its slightly dangerous)
coming to mbuna, only labidochromis (specially yellow labs) and cynotilapia (afra) can easily be kept there. i'd stay away from the other pseudotropheus (saulosi is one, but its a dwarf, other dwarf ones like demasoni are lot more aggressive), melonochromis and labeotropheus ones....
but at the end of the day you have to do your own research and make your own decision. i have found out advice doesn't get you through everytime.
good luck.