All dwarf gouramies are the same species (Colisa lalia) so the color of the fish is irrelevant.
Male dwarfs, as was said, are territorial fish. When they are kept together, particularly in twos where the aggression can't be split between several fish, one will tend to be dominant over the other and can realy stress the subordinate male out if the tank is small.
While 29 gallons is a good size for dwarf gouramies, you ahve to consider that the one you already own will be settled down and have established the tank as 'his' territory. Any newcomer gouramies will be treated as 'intruders' and they won't have much of a chance to settle in. This is extremely stressful and, particularly with fragile, vulnerable fish like dwarfs, can lead to serious disease and, potentialy, death.
You can sometimes get round this by re-aranging the tank's content just before you add the new fish. That way, the established individual also feals as if he is a 'newcomer' to the, now unfamiliar, tank. However, this doesn't always work long-term and you still have the problem of having only two (so aggression will be concentrated on the less dominant fish as opposed to being split between several less dominant individuals).
What I'm getting at is that, if you realy want some dwarf gouramies to complete your tank, you could get yourself a couple of females. Females are not territorial and will be welcomed by the male (though he will chase them around) as potential mates. By having two, the male's attention is split between them so neither becomes particularly stressed. re-aranging the tank before the addition of the females is still a good idea as it'll give them more of a chance to familiarise themselves with the new environment before the male makes an impression on them.
The obvious downside to females is that they are not colorful and that they can be difficult to get hold of.
As far as different species of fish go, I'd suggest some colorful livebearers - a trio of swordtails or platies for example (avoid mollies though - they tend to dislike gouramies). For something more 'unique', perhaps a pair of rams or a trio of american-flag fish. There are also many mid-dwelling species of gourami you could try - the most obvious being the croaking gourami and sparkling gourami. Both species do well in groups of 3+ with 2 or more females per male. Croakers grow to about 2-2.5", sparklers to half that. Because of their different habits, they won't clash with your existing dwarf gourami.