Right, color is for mood. It's emotional, and of course it can be used for other reasons. Place some blue in the back of a landscape and it leads the eye into the horizon. Color is also used for composition. A highly charged color like a bright red (more emotion....blood) can grab the eye's attention, so even though you always have to observe formal rules of composition, you also have to be aware of how color can change/modify them. A little can go a long way. The colors of the American Southwest are memorable for their beautiful soft palettes, not found in other areas of the country. A little study on color and how it affects humans can be very rewarding if you shoot or otherwise work w/ color. It's actually one of the reasons I shoot B&W. You're forced to use basic good composition and have a great subject w/o the help of color. For painting, I would never dream of working w/o color, for etching, B&W is the ticket.

One thing to keep in mind: there is no such thing as natural color. As clouds go over a vista, shadows fall on objects, the sun goes to a different angle, etc, colors will go all over the place. Color is only relative, not absolute.