It's variable. I have chicks that have different tail lengths at different ages. The stages of growth are not the same between individuals unless there's a very high degree of inbreeding. Often you don't know for sure until they get their adult feathers.
Tails are something I've been focusing...
Okay, I didn't think so originally but wondered because @I Like Turkeys said the males are supposed to be red and females pale. Which could be "pale red" aka lack of testosterone , or I thought perhaps with your addition of plumage specific SOP it might be one of those genes affected by...
I totally agree with @Amer , they can hatch with yellow *looking* feet... but the yellow I've gotten used to spotting on our Mosaics, the kind that sticks around, looks like a surface pigment. It has a shine to it. I don't want green legs in the Mosaics so I've been trying to breed away from it...
I see yellow on her legs. With yellow skin over slate (blue/gray), that makes green legs. They will likely show better as she grows. So, I would say, half Ameraucana.