Originally Posted by Dreamcatcher Arabians
You pretty much have a 50/50 chance of throwing a grey foal. It may not be grey when it's born but could start to white out as it got older. I would not breed to a patterned horse with a grey mare, not if you care about color and pattern.
I did this probability with her base as chestnut (because of the 'flea bites') if she was born bay, then it would be a little different but not much.
Offspring Color Probability
Gray (Chestnut) Overo
This is incorrect. You have no idea what color the mare is underneath the grey. She could be anything based on the information we have, which means the resulting foal could end up any color. The most I can tell the OP from the information provided is that the foal has the possibility of being chestnut, but if it ends up black-based, it will be heterozygous for black (as chestnut is homozygous red), it has a 50% chance of being a frame overo, and a 50% chance of being gray.
Does your mare's AQHA papers say what color she was born? Or is she registered as gray?