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.
J. Zheng, S. Pawar, и D. Goodman. (2017)cite arxiv:1710.04626Comment: Submitted to IEEE Transactions on Visualization and Computer Graphics on 11/04/2018.