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.
A simple guide to plotting maps in R using the High-resolution map data of the World. Examples are given for plotting the whole world, a selection of individual countries, a map of the United Kingdom or adding locations to your map.