This explains several ways to use consistent colors across plots (same way works with scales). However, the easy way explained in one of the alternative answers does not seem to work for some versions of R/ggplot2. Opting for the accepted answer always works though.
K. Agarwal, C. Dixon, E. Rozner, and J. Carter. Proceedings of the Third Workshop on Hot Topics in Software Defined
Networking, page 157--162. New York, NY, USA, ACM, (2014)
W. Duch, and G. Diercksen. The Journal of Chemical Physics, 101 (4):
3018-3030(August 1994)Multi-reference form of the Davidson correction given here..