There really should be a legend somewhere, but in practice the amateur astronomy community has standardized on a convention that's more or less the same in color scheme and scale to the one reported here: https://djlorenz.github.io/astronomy/lp2020/colors.html.

You'll see the same scale in use at other light pollution sites, such as:

http://www.cleardarksky.com/maps/lp/large_light_pollution_ma...

https://www.lightpollutionmap.info

https://cires.colorado.edu/artificial-sky

Was also looking for a color legend. I can generally understand the different colors/layers, but was initially confused.

Text that explains exactly what each color refers to would be helpful.

You're right it's missing. It shows the Bortle class of each location, see https://www.lightpollutionmap.info for a better explanation.