Canary greenfinch cross? (1 Viewer)

Hi - I posted a picture here recently of a yellow and white bird which you all very kindly id'd as a greenfinch with perhaps an excessive yellow gene. Now, I am pleased to say there are some baby greenfinches on the scene and at least one (maybe two) have the same yellow and white markings only. The other ones seem to have normal colouration. Because not all of the babies have picked up this yellow I wondered if there was a cross with a canary somewhere (this was suggested). However, I know nothing of bird genetics so perhaps you could give me your ideas on whether it is caused by a genetic mutation or a cross breeding somewhere back along the line.
Here is a picture of one of the yellow babies.

I think the fact that it has bred shows its a Greenfinch!

Yep,
and size and general structure of the bird show that it is a greenfinch. Canary would be slender looking with a smaller bill; and the hybrids CanaryXGreenfinch would not have the yellow greenfinch markings on the wing as clear as that bird. Probably both Greenfinch parents had the same recessive gene for that mutation.