Hehe, there’s a default tool in Paint Tool SAI (assuming you happen to use that) called ‘Water Color’ that I often use!

And these are my usual settings for it!

(Though, I do often mess with the density, depending on just the right consistency of blend I may need!)

And in terms of colors for shading, I’d say it depends on the mood of the picture! If there’s a more sad, dull scenario taking place, being somewhere in the blues at a slightly more gray hue could do! If more vibrant and happy, reds/oranges/yellows could do!

It also depends on the environment at well (day, night, raining, sunny; that sort of thing)! It also doesn’t help to experiment! (I’ve even seen people give tips that using a color that’s opposite of a character’s overall color can give interesting contract! Like, an overall character with blue shading/lighting, a pink with green, etc.) :)

It’s usually just fun to mess around with your settings and see what happens to work best with your shading methods! At least in my book! <3