That's a good idea. I see one problem, which is that is most females of most species (including RWBL) are brown. Therefore, adding brown as a value for the primary color search will INCREASE the number of possible matches, rather than DECREASING that number. I think the intention of the search engine is to narrow the number as quickly as possible, so using the males for searching by color is generally more effective.
By the way, if you already know what bird you want to find, but want to find it using the search engine, look at the Identify page. For Red-winged Blackbird, you can see the Body color is described as black, sheen or iridescence, so entering those values in the Color Primary search attribute will include your bird among the other candidates. I don't see secondary colors listed on the identify page, but if you add red and yellow to your search, the number of Matched birds goes down very, very quickly, so those colors are clearly referenced in the database.
Hope this helps!