Getting your whitebalance just right...

That's something often overlooked by the beginner and too many intermediate photographers. Get it just right and your pics will look that much better. Get it wrong and you'll get pics that just look "Wrong". Get it really wrong and you get nasty red or blue casts.
Many current DSLR cameras feature autowhitebalance which is mostly ok between 3500 and 6000 degrees kelvin. And they have presets which almost never match what you're doing. Get a bit of practice and you can guesstimate light color.
There's ways around this of course, the easiest is to shoot a custom whitebalance from a piece of plain white paper. Or buy a special test card. Or spend a lot of money on a special lenscap.

You can also really dive into the matter which is what Leon Goodman has done. I've not read the entire article with care yet but at a