Found this growing among normal dandelion plants I grew from seed, it's saved in my plant pots so next year should bloom.
It sounds like you buy dandelion seeds for greens, therefore I wonder if it is a seed of lettuce variety- or if it is crossed with a colorful lettuce grown at the seed source.
Dandelions can cross breed with lettuce? Some others sprouted dandelion flowers and except color the purple one matches their leaves identical so I still think its a dandelion but my question is what variety grows purple leaves.
They are both in the Aster family- I am speculating that they could cross. In restrospect, that was probably kind of silly of me. Maybe a better point is that being in the same family as lettuce, chicory etc it is not so surprising that a dandelion could have leaf color.