Regarding point #2, I've found it usually balances it a bit more and while keeping the same values, makes it more colorful. If working with a limited palette, a neutral grey can be used dually with warm or cool colors on the object to cast an opposite cool/warm shadow - which usually looks quite complementary, although I can't recall seeing a warm shadow cast from a cool-colored object very often - seems unnatural as shadows are generally cooler.
http://androidarts.com/art_tut.htm - this link touches a bit on color theory from a digital-painting standpoint, though it's pretty applicable. Color and Light for the Realist Painter by James Gurney is also a good book that has some info about creating unified palettes.
We find it pleasing to depict shadows as slightly cooler because the shadows we are most familiar with behave this way; those cast on a sunny day outside. Daylight from the sun is a strong source of yellow (warm color) while the remaining sky is a source of blue (cool color), and it is the sky's ambient light that you see when see a stark shadow cast by the sun, which makes the colors seem cooler in contrast with sunlight areas.
http://androidarts.com/art_tut.htm - this link touches a bit on color theory from a digital-painting standpoint, though it's pretty applicable. Color and Light for the Realist Painter by James Gurney is also a good book that has some info about creating unified palettes.