In answer to your question, someone once bred a white bellied to a black capped caique. They ended up with chicks that looked like a mixture of the parents... however some looked more like dad while others looked more like mom. I don't know WHAT they thought they were breeding because they wanted to know what chicks they should pair up with new blood to make their offspring (thus 3rd generation chicks) appear more like one or the other of white bellied or black capped.
I think what they did not realise is that they allowed for hybridization to occur between two different species within one genus to occur, NOT a "breed" or "mutation" within one species (such as the various colors of budgies, or that all dogs are one species thus it's not called hybridization when you breed two different dogs together... you create mutts... which in turn were created from standardized mutts which were then classified as 'purebreds', if you think about it).
As far as the foot thing, well I couldn't tell you! It may be hard to tell a hybrid from a pure if they've got very little blood of the other species within them... and when breeding two different species together, there is something like 1/3 of a chance (probably less) that you'll get a chick that appears like one of the parents... such as some blue and golds, you can tell they are hybrids when they appear a little more green than they should, or the fact that they've got a bit of horn coloration on their beak... maybe their golden breast is a little more orange than yellow. Such as this catalina...
I've actually seen a hybrid B&G (in photos) that was sold as a B&G however I think one parent was a greenwing, thus it was a harlequin? (could be mistaken on what other parent was). However,the yellow coloring was nearly identical (if not identical) to that of a B&G, however the bird was a little greener around the body than it should have been and it had a bit of horn coloring. So in short, you may never really know if a bird is a hybrid if it appears like a normal.... and in some cases, hybridizations are done in order to create mutations in some species (that I've heard of... such as some eye-ring lovebird mutations and alexandrian parrots).