i have always wondered about, why good artist first makes a base color, not to be anoying to hear at it is really beautiful, im just really curiouse, about what it is for ^^ sorry for sound stupid!!! i really not trying too!!
Base colors are good for defining foreground from background, or to get a rough idea of what color an artist is looking for, or to see where the shading would go. They have a lot more uses than what I said, but really, base colors are the best thing in art since a canvas XD
Im pretty sure its just to make the coloring WAY easier, cause you can just drop a "clipping mask" above the layer with the base on it and set the "blending mode"(button next to opacity on layers window) to color and you can color in your stuff without worrying about staying in the lines. it DOES cause a couple complications with the colors though. for a more detailed explanation youtube search "Understanding Colour and Value" and look for the one by Sycra.
On another note, i would also like to add that ive never really used this method so it would be smart to just check the facts and look into this a bit more if your interested.