I take it this is a male betta? If so, nothing really- male bettas can be particually agressive towards other fish that are colorful, and don't really do well with ones that are active as male bettas are easily out-competed for food.
If its a female, it be a different situation, but how many do you have?
Mollies, platies, cories, and loaches aren't colorful enough?Sorry, couldn't resist. How long has your betta been in with the others? If he's been okay with them, then I don't see a problem in adding rasboras, neons, danios, cherry barbs or even a pearl gourami.