Filled Map Colour Saturation With Legend

Within the filled map visualisation when you add a legend, you cannot then add any colour saturation to the legend. So for example if I split the map of Spain into three areas using the legend option, I have three different colours for the three regions. What would be useful to add would be the ability to have different colour saturations for each legend (i.e. region), for e.g. if the west region is coloured yellow, you can have different saturations of yellow depending on the sales within that region.