Hi! Sorry for the late reply orz. I was busy at school, but now I’m free :D
First, watercolors are water based (why did I even say that) and are transparent and delicate in nature. It’s important to know the nature of an art material so you can get an idea on how to use it well and basically that’s your first step - understanding your art materials.
Now onto the main point, about coloring with watercolors. Let me take out our subject of this tut, Chris.

Okay, so here’s our lineart, and until now, I’ve just colored his hair and diseased skin (need to fix that lol), but I think it’s enough for this tut. Usually, beginners would just slop on the colors, as if base-coloring in Photoshop or Sai, but that’s not how to color with watercolors in this case. Instead use a clean brush and dip it with water and slop it over the area you want to color - to create a thin layer for coloring. Kinda like using the selection tool or the magic wand in Photoshop or Sai, like this:

Then pick any color you like and dap your brush onto the shadow/dark areas. After that, clean your brush and then fade the colored areas, but try to preserve the gradient between the colored areas and the previously uncolored areas, kinda like:

Well, in the photo, I first dabbed the colors on the tips of his hair and faded the colors into the centre. And that’s what it looks like when you’ve finished the “base coloring”.
Of course, about shading, I like to shade semi cell, and so I have to wait for his hair to dry before I can do anything else like add in darker shades, but when it comes to blending in colors, of course it’s easier blending on wet, but be careful. Make sure the surface you’re working or your brush on isn’t too wet or you’ll get ugly-looking “flower” blobs instead of even blends.
Also, in some watercolor works, like Emperpep’s, you can observe a thin white gap between subjects, and this is done to prevent the colors from the background and the colors of the character from mixing together, and when that happens, it can indeed be ANNOYING, and speaking of annoying, it’s not nice to ruin a picture with blobs of water on it isn’t it?
When I color something wrong, I usually clean my brush then scrub out the mistake in vain (I’m serious) and it doesn’t turn out good and even though it’s basically “clean”, you still have this blob on your painting surface and that’s why I said fixing mistakes can be annoying. Also, if you don’t clean your mistakes carefully, you could even affect other parts of your picture as well, and it sucks.
I hope the tutorial helps! :)
Panda