Well I think the basic concept is done now. Let's do a little bit of atmosphere. For this I recommend to use Soft basic round brush with such settings. + set overall brush flow to about 10 -12 %.





Use separate layer for this, so create one and GO.

Result:



If you are satisfied with the results ? merge down this layer (Goal: only one painting layer). Now I will use Dodge and Burn tool to fix some value (foreground ? Blacker, background ? lighter) so here is our result ? for this operation I have used the same brush settings as for the Atmosphere but only with angle jitter is set to 0%.

Now let's try to put some loose color like this (same brush settings):



Then I have decided to create a Posterize layer (value -19-20)- set the mode to Hard Light.



The comes the gradient layer (Color dodge) + master Opacity/Fill ? 45%/75%.