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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.
Jun Zou, Faramarz Fekri, and Steven W. McLaughlin. Proceedings of the 2015 IEEE/ACM International Conference on Advances in Social Networks Analysis and Mining 2015, page 1586--1589. New York, NY, USA, ACM, (2015)