Can frozen (fresh) flower petals be used in baking the same as unfrozen fresh petals?
If you're baking them I think there is a strong chance the color will change no matter what. But you might consider sugaring them first.
To do that separate an egg or two and whip the whites to frothy. Paint the petals with the whipped egg whites and coat with superfine sugar. The trick here is to reduce the water in the flowers as well as the water activity in the eggwhites. What I used to do was set the sugared flowers on a cooling rack. When done with all of them I'd dump them into a locking plastic rectangular container. Basically an upside down Tupperware cake container. It had about 1/2 inch of sugar in the bottom and the flowers on top. I put them in the fridge. I knew others that left them out still others just put the sugared flowers on cookie sheet and left them in warm but off oven overnight. I'm not sure it really matters.