There is a real and logical way to explain this. (I hope we can pick multiple leaders per civ and each would give a different bonus.)
IMHO there are many ethnicities of humans and every species on earth. The altarians are not different, in one area they are white another they are blue. It's even happened before on earth when a family had abnormal blood flow in there bodies causing a lack of oxygen in the blood making a blue tint.( I forgot if the weird genetics were because generations of insest or not but feel free to google it.)
1 lack of oxygen in the atmosphere in an area of the planet due to high altitude
2 the food on one of the continents were they evolved would be a major factor.
3 metal content. Specifically neodenium* (I probably didn't spell it right, but I was referring to the element.)
4 or something to do with the precursor presence on the planet specifically it might have been in one area but as they moved through the planet and away from the precursor area (which made them blue.) the altarians lost the tint.
5 radiation is different at different hemispheres and they might have absorbed another part of the light spectrum.
although they are all my ideas 2 is the best option to explain the difference in color.
nothing should be permanent at this point so the tattoos can still be added. And since elsys mue is dead. a race specific super project should be made to honor her leadership and liberation of altaris from the drengin menace.