I don't think it would be darker but rather adding a bit of light blue... of course others could give better advice I'm sure.

then judging by that i guess that this one sand dune at the upper right should be darker right?

Actually, the reason for this is quite fascinating.  In the visible spectrum the different frequencies of light diffuse less or more readily depending on how close they are to the blue side.  Blue diffuses very readily and thus when you see a blue sky it's because the blue light from the light source (the sun) is being scattered by the particles in the atmosphere and thus reach your eyes.  Also at sunset the blue light is scattered once again (away from the viewer's eyes) by the obstructing particles resulting in an absence of blue and more noticeable red.  I also hypothesize that this is the reason why, when coloring a white shirt, you should add blueish shadows rather than grey. </geek speak>

Just thought it'd be neat to have a reason along with kuph's explanation 

EDIT: I meant diffuse... diffuse! ugh... fixed

...wasnt expecting that as an explanation but that was cool!

!!! that was cool man!

No, the dunes shouldn't get lighter with the distance.

It's all a matter of athmospheric perspective: the "layers" of air between the viewer's eyes and the viewed object make it look  more blue, sometimes grayish. Like in this picture:  www.destination360.com/north-america/us/tennessee/images/s/great-smoky-mountains.jpg : you know the back mountains can't be blue, they're covered in trees. But they look blue anyway because of the distance.

Here's what I could say about the colors of the dunes. To sum it up, the further an object is, the more it's color will tend to blend with that of the sky.

You're kidding yourself if you call that thing a rock 

hmm.... perhaps i shal improve it but i don't want to take the building/rock away. i like th building/rock. Oh and.....do you think the sand should not get lighter when further?

its pretty much a bunch of dunes with two suns and that arab-styles building.

aside from the twin-suns, this isnt far from what one would see on earth.

I like the piece but that ward rock is looking a whole lot like a building to me.  I would remove the building rather than try to slib by the rules with the rules...  As the hills progressively get farther away, they get lighter.  Don't really understand that.

Would really like to see this in the challenge ;)