Mark
As I think about this and my experience with engines over the years I have my doubts that a paint will be found that will hold it's colour as temp's go up. The guarantee on high temp paints is that it will not crack, peel, bubble etc under the given temperature. In my automotive experience the first area we would see and engine discolour is on the heads, around the exhaust manifold. If you want to go way back it would also show on the intake manifold if the vehicle had a heat riser system to heat the choke coils. All heat related.
So having said that, if the paint does discolor a bit on those sandys that get ridden I don't see it as any more a big deal than the exhaust discoloring etc. It just shows you ride your bikes. Yep, would be nice if they stayed perfect but they don't. I justwent and look at my Harley. The grey is actually changing slightly on the head vs the rest of the engine. I will be watching to see if you come up with a better product.
