Koa is generally reddish gold to dark brown in color. Its acute interlocking wood grain produces a highly curly or figured appearance. Its origin is always Hawaii. However, Koa wood's appearance can be highly variable.
Koa ukuleles are rarely mass-marketed and typically produced as limited editions or special models. Some brands try to confuse the customers using Koa terminology when the wood truly is acacia (from the same family). Be cautious when you see a cheap koa ukulele, it is most probably not koa wood (Mr. Mai and Kunde Brand typically do this).