Humans are very good at looking at a scene under different types of the light source and judging what is white. Cameras aren’t so good at that, and will often produce a photo that has an unnatural look, with a blue or orange color cast to it. Here’s a quick and abbreviated technical description – feel free to jump ahead if you like: the light has a temperature, measured in K (Kelvin). 5000-5500K is (roughly) neutral, or daylight, 3000K is shifted towards blue, 8000K is shifted towards the red.
Here’s an example. Below is a photo of a beach (Cambois in Northumberland). It was a fairly bright day with a little cloud around, so I set a white balance of 5500K when I took the photo:
If the white balance on my camera had been set for a color temperature of 3000K, you’d see a color cast like this:
White balance set to 3000K
And if it had been set to 9000K, it would have looked like this:
White balance set to 9000K
Most of the time, the Auto White Balance (AWB) on your camera will do a decent job of picking the correct white balance, but if it gets it wrong, then it’s useful to know it can be changed in software (a quick note here – RAW format is beyond the scope of this web site, and most cameras can’t shoot in RAW anyway, but it gives much more flexibility when changing white balance in software, basically because it doesn’t actually set it in camera. If you have an SLR or high-end compact, you should look into using RAW).
If AWB doesn’t cut it, you may have preset white balance levels on your camera (they’ll range at least from something like tungsten at about 3000K to shade at about 8000K).
If you do get a photo where the white balance is incorrect, it’s possible to change it to some extent in software.
How to do it in Picasa
Open your photo and click on the second tab on the left (finely tuned lighting and color fixes).
Picasa uses the term color temperature, and what you need to do is move the slider left to decrease the color temperature (move it towards the blue end of the spectrum) and right to increase the temperature (move it towards the red/orange end of the spectrum).
Again, be careful with this – it’s easy to go too far one way or the other.
Click Apply when you’re happy with the results
A final note – “warming” landscape photos (moving the slider right) can be very effective on sunset or sunrise pictures:
Tips: If you confuse about editing white balance saturation or making mistakes, you can check my previous posts about “specific mistakes made by beginners in photography”