I am talking about the white balance, of course :D. If you're familiar with ACR (Adobe Camera Raw) or other RAW processor, you'll know that there's a Temperature slider and a Tint slider.
I've prepared a few seriously exaggerated samples for the ease of explanation. First, the Temperature slider. At this point, we leave the Tint slider as it is.
White Balance As Shot
Too Warm. Happens if you move the slider too much to the right. You'll get somewhat similar result too if you shoot under tungsten light with AWB.
Too cold...notice everything becomes bluish? Even the green leaves. Also note that the water lily petals have clipping in it i.e. loss of details and/or textures in the petals. AWB under fluorescent light will give you similar result.
Now the Tint slider. Leave the Temperature slider at As Shot position.
Too green if you move the slider to the left. Green cast in everything.
Too magenta if you move the slider to the right....
However, all these won't mean a thing if your display is not calibrated. I use Spyder2Express to calibrate my display at least once a month. How you calibrate your display is crucial too. Make sure there's no backlight or better still, have it done with shades down and lights off.
Even though my display is calibrated, I try not to comment on WB unless it's really really really way off.
If you shoot in RAW, it's quite easy to correct the WB. Just play with both the Temp and Tint sliders until the image looks right to you. Or if there's a white area in the image, use the White Balance tool and click on the white area and that should get you to the ball park. You can then further fine tune from there. Theoretically, the WB Tool works best on an 18% grey area.
However, in the real world, you don't always find a white spot or light grey spot it's back to manually adjusting the sliders until what you see is what you like :D
In my 40D, I have the White Balance on Custom, manually set at around 4600K. Seems to work much better than Auto WB.
I use Spyder3Epxress for my display calibration
If you shoot with natural light, it will help if you use a white balance cap or white balance card to obtain more accurate white balance.