My first question would be what kind of clowns?
If they're ocellaris, percula, or skunks, you've got lots of options. Some of the mellower damsels in the genus Chrysiptera, such as the azure or Talbott's damsels could work, as might a tailspot blenny or shrimp/goby team-up. A royal gramma would probably work as well, and certainly qualifies as colorful!
Do keep in mind that as the clowns grow, they'll pair up, and their demands for territory will become stronger as they do.
~Bruce