Look at my Mario Bros. cake in my photos. The blue stars with the letters on them. I started with white gumpaste and added the powdered CK Royal Blue. The longer it sat, the darker it got. Prepared it the day before I needed it. I could have gotten it darker by just adding more. Good luck!
I don't know what colors you have onhand to start with, but you can add black to royal blue to get navy.
Also, if you're making MMF, you can tint it while melting the marshmallows and before you add the PS if you'd like to save a lot of time trying to knead it in.
Quote by @%username% on %date%
%body%