Great question, Becky - the reason is that indigo with brown or ash-red doesn't really make much difference. With ash-red, it basically just looks to be an ash-red; with brown, it basically looks to be brown with a tinge of "something" over it. That's why most don't bother breeding the birds that way, it doesn't provide anything in the way of spectacular color. It does explain though why indigo is sometimes carried along in families of birds that don't have lots of blue and/or spread in them if they are basically brown or red or ash-red families and then suddenly a "weird" color pops out when those families are crossed to blues. In fact, that's pretty much how Levi and Hollander recognized it as something unique when it showed up in Carneaux crosses. It was occasionally seen before in European and Middle Eastern breeds, but these two guys recognized it as something genetically unique and figured out just how to breed it to produce the "andalusian" color at will.
Frank
Frank
