It can be any fill you like.

If you want to get a nice hand drawn feel to it all, though, you would do very well to use a parchment or paper of some kind. The grey in the parchment would subtly tone down the glaring electronic "ouchy-ouch-ouch-that's-so bright-I-can't-see-what-colour-it-really-is" in any colours you add on top of the map as a whole, and bring everything together nicely

Adding colours like faded watercolour washes is very simple once you've set the file up to multiply the background over the top of the map. You just add another sheet on top of that again, draw solid fill colour patches on it (use a separate sheet for each colour), and add Edge Fade Inner and Blend Mode effects (Blend mode preferably set to Multiply again to prevent the colour from being paler than the paper), and away you go