The colors are not arbitrary, as they give the radioactivity (?!) order in those units. Probably the radioactivity of the surrounding waters is modeled as 1 (or less), and they input the released data in multiples of background radiation or "legal limits". The map then does give information about modelled radioactivity levels, as the difference between 10^6 and 10^0 is significant and clear in whatever units. But of course, putting "Arbitrary units in the legend makes a silly impression.