You may group your cities and refer to the grouping: IF(continent = 'EUROPE', BLUE(), IF(continent = 'USA', RED())). You may also load a table with countries and desired colours and refer in the calculation to it. Or have I understood you wrongly?
I like the idea to create a table but I don't know how to assign color for each DEX (is my dimension) in the background. I tried all...
Load * Inline [ DEX, COLOR AJACCIO DPC,red() BORDEAUX DEX-ELP SUD OUEST,blue() LYON DEX-ELP CENTRE EST,green() MARSEILLE DEX-ELP SUD EST,yellow() PARIS DEX-ELP ILE DE FRANCE,black() RENNES DEX-ELP OUEST,lightblue( ) VILLENEUVE D ASCQ DEX-ELP NORD EST,cyan()];