With dogs and other mammals you can get genes that produce various hair colours that can produce all sorts of mixtures. With budgies the genes tend to alter either body colour, or markings colour or markings pattern etc. This means you usually only get one body colour at a time rather than several mixed up.
What you get from two normals depends on what genes they have (particularly the recessive genes that aren't visible - that's where the surprises come from). Knowing which genes are recessive helps a lot, for instance all the sex-linked genes in budgies are also recessive.
All greens are dominant to blue so some Grey Greens could be split for blue but only if a parent has passed on the blue gene. They don't automatically have the blue gene as green is the wild type, the original colour, with blue being a mutation, - often breeding is the only way to tell especially if you don't know the parents and their history.