I made a fondant cake with purple accents and the purple turned blue gray.

How can I quick fix it?

Purple does tend to fade, especially in bright sun.  Try painting over the accents with some food coloring or a food coloring and vodka mixture.  

Next time, if you add a little pink the purple will hold its color better. I would just try to paint over it, with some color gel watered down with vodka or lemon extract.