Step 1, reference picture. Google is your friend
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Step 2, determine colors. From this reference pic I can see Dark Flesh at the darkest parts, and vomit brown at the lightest.
Now to paint it! for a 6x4 board I'd probably suggest priming it then basecoating with the games workshop "airbrush" to save time (and if you spent $300 on the board, the $40 spray gun and propellant shouldn't be too bad
). Then drybrush it with successive layers of dark flesh mixed with more an more vomit brown.
Details! There are secions of "rocks" or slate on the board, they should be painted with a contrasting color or at least something that stands out. Possibly a grey or a more natural shade of brown. Then there are the skull pits. Make them necrons! As mars is a tomb world. Boltgun, wash black, chainmail drybrush. And if you're feeling creative, fill them with green tinted water effects.
Now for that extra little effort, I would go by a hobby shop and pick something up for mars dust/rocks. perhaps something like this:
http://woodlandscenics.woodlandscenics.com/show/Item/CF-FM/page/1
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And if you can find it perhaps a red fine turf. OR you can use red weathering powder, spread it around, and seal it iwth water/white glue in a spray bottle. Haven't tried it personally but that should clump up to make convincing mars landscape ontop of the colors selected above
