You make it a bit more complicated than it is, because the order of the colors doesn't matter, blue/yellow is the same as yellow/blue
So the potential pairings for a blue/green x yellow/red are:
blue/green
yellow/red
blue/red
blue/yellow
yellow/green
red/green
purple
diamond/Infinity
so basically any dragon except diamond hybrid or a magic hybrid (or the light dragons in the new dawn version)
But you can get both colors from only one dragon, which makes it that much harder to narrow down the possibilities :/