Before people start going too far with the population density layer, we need to consider how we want the layer to work? What does it even mean that a pixel is red? How many people live per square km or whatever?
I think we need to decide that, and that we need to pick a spectrum of colors with some defined ranges for the colors. The current layer doesn't really hold any other information than "I want a lot of people to live here". It doesn't inform you about the actual population density.
Did we ever decide the radius/size of our planet?
http://en.wikipedia.org/wiki/Population_densityIf we intend there to be roughly the same number of people on this planet as Earth, I'd suggest these colors, the numbers to the right being people per square km:
Black (RGB 000,000,000): <1
Blue (000,000,255): 1-4
Cyan (000,255,255): 5-16
Green (000,192,000): 17-64
Yellow (255,255,000): 65-128
Orange (255,128,000): 129-256
Red (255,000,000): 257-512
Fuchsia (255,128,255): 513-2048
White (255,255,255): >2048
Also I think for a sort of similar reason that the coast part on the coastline layer is problematic. To me it would make more sense to extend the height map to the sea, so it includes a sea depth map, that could display shallow areas like the coastline color. The current coast color doesn't really say how shallow the water must be to be considered coast or not.