I have used 12 gauge wire on a 15 amp circuit, (I had lots of single strand 12 gauge wir e). I am running the wire through emt.green I ran out of green 12 gauge, but have plenty of green 14 gauge, Can I use the 14 as a ground with the 12 hot and nuteral?

It will be a code violation. A very minor one IMO, but a violation nonetheless.

If it were me I would not worry about it.

Why not just use all #12 and use a 20A breaker?