I have Stanley, Santa Rosa and Green Gage plum trees. The Green Gage used to have green plums but last year they were purple like the Stanley beside it. Will the two trees cross?
You mean will the Stanley have an affect on coloration of the Green Gage? Nope.
Agree with HM.. you might have another plum on the same tree.
Cross pollination affects the seed not the pulp. That's why hot peppers can pollinate sweet ones to have heat or white can make yellow corn white and yellow.