Green legs is simply the visual effect of yellow skin being mixed with another set of leg pigment to create the "green". The same leg pigment on a white skinned chicken creates blue legged chicken.
So.. mixing certain breeds almost inevitably results in green legged birds. An Araucana or Ameraucana (slate/ blue legs so they have the "leg pigment" plus white skin) mixed with RIR or BR or anything with yellow skin will inevitably result in green legged chicks either in the cross or somewhere down the line.
Never thought about this until now, it does seem to be very hard to come up with a breed allowing for green legs in the standard.. Interesting. Wonder why that is? (tiki is correct as for some Modern colors allowing for green legs). I quite like the look of green legs too..
Green legs are also common in Old English or pit game stock, again due to the leg pigmentation and yellow skin genes being widely present in these birds. Many backyard mixes out of miscellanous breeds can have several green leggers.
There is a green legged chicken. She is correct.
Zielononozka Polska. I do not know if there is any in the US or not. It is a Polish breed. I spoke to a gentlemen on the phone from Poland who is looking for some. If anyone knows of any here in the US please contact me.