It's almost impossible to separate I. versicolor from I. virginica just by looking at the flowers. Generally, in the field they are best distinguished by looking at the seeds or the inner wall of the seed pods. I. virginica seeds have a corky seed coat, while those of I. versicolor are relatively smooth. Similarly, the inner wall of I. virginica seed pods are a bit rough, while those of I. versicolor are smooth and shiny. Even those characteristics can be subjective, though, and it's often difficult to be sure which species you are looking at.
They have a different number of chromosomes (72 for I. virginica and 108 for I. versicolor if I remember correctly) so a DNA test would give a definitive answer, but that's obviously not a very satisfying answer, sorry.
