I am working on a white buttercream graduation cake with navy and red fondant decorations.  How can I make sure that these don't bleed into the white buttercream?

you could coat the back with corn starch or white chocolate but before all that test it on your buttercream -- should be fine -- you could do white fondant pieces and paint them in the colors you want for more control --

you need to be sure to make any temperature changes gradual -- when taking out of the fridge keep it boxed till it warms up closer to room temp -- but really you should be ok with those colors -- be sure cake never sweats --

You could make your decorations from modeling chocolate instead of fondant.  The color doesn't bleed from modeling chocolate.