Interpolating (hex) colour values

In visualizations, it is often desirable to be able to display gradations of colour, or to interpolate from one colour to another. When plotting data from a dataframe, we would like to be able to do this automatically from any one colour to any other colour. For instance, if we want to plot a number of distinct colours but want the transition from one to the next to be gradual. This post shows how to do this easily and neatly.