Hi Taz - Let me start by saying I'm far from 'expert' but I'm sure others will correct me where I'm wrong!

As I understand it, cross-pollination wouldn't affect this year's flower colour, only the seed DNA will be changed -It's more likely to be an individual 'sport' associated with a random mutation in the existing plant.

Given that, vegetative propagation would probably be your best bet as that will preserve any genetic changes from the original plant: it's late in the year, but take a cutting with the sport flower and including a bit of root and plant, keeping it *just* moist and cool but frost-free over winter. It should start to take off again in early spring. The sport may, or may not, appear again! (But try from seed as well, apparently it's an easy plant to raise, and you never know ...).

Hope that helps (it's what I would try!) - Anyone else want to correct my genetics/plant-care please?

With growing on seed from cross pollinated plants all you can do is try it and see - you might get several different ones from the same batch of seed. After all, that's how new varieties sometimes appear. And you don't know which of your geraniums has pollinated the cross - if it was Rozanne again you might be in luck and get a good one. So yes, propagate vegetatively if you want to be sure, but grow the seeds as well just to see what comes up. And let us know!