"Rainbow" is the term used to describe the colorful streaking that sometimes appears in yellow poplar wood. The cause of this discoloration is not fully understood but scientists believe that is has to do with the mineral composition of the soil where trees grow. The hues can range from jet black down through purple, blue, brown, gold and green.
Still seen as undesirable by large production shops because of its inconsistency, rainbow poplar provides a way to get exotic figuring and colors from a domestic wood for those willing to take advantage of its abnormalities.
The wood itself is very soft for a hardwood but dimensionally stable and easy to work with. Poplar trees grow very tall, wide and relatively fast, so they are a great renewable resource. In short, if you are looking for unique figuring without paying a premium, rainbow poplar is as good as it gets.