I m not going to go into the now sixty year old history, but if you re interested this effort is describe in Levi s book, The Pigeon . Suffice it to say, though, that indigo was found to be non-sex linked dominant in its inheritance. It is not quite a simple dominant, however, and this is why it often goes unrecognized even in racing homers, where I ve seen it in direct imports from top Belgian lofts. It is a deep indigo blue coloration all over the bird. In fact, indigo will be almost invisible on the bird.
Brown indigos are simply sort of yucky looking.
Normally, the black tail bar is washed out, the checks and wing bar are changed to a bronzy indigo coloration and the blue body color tends to be a bit darker. Homozygous means a bird carries two genes for the trait, rather than just one. Some people get confused here and believe female pigeons can t be homozygous for any color modifying trait. That s incorrect. I ve seen examples that looked just like ash-red checks or bars.