Originally posted by tuco My most used filters are yellow and orange. I use the yellow the most. It only costs me 1 stop of light and it's a good general purpose filter. In a scene of multi-colors it can make a difference. A lot of common colors reproduce as or near a middle gray. A yellow (and orange) can often help break that up into more contrast by lighting their color and darkening the opposite color with a varying lesser effect of colors in between.
Edit: Here are two examples of a yellow. In the first one, you can see it put some tone in the blue sky even looking into the sun. It's not as deep of an effect as a orange. And the second one is looking into the sun again in the late afternoon with a faint, bleached out blue sky but I still managed to produce a tone other than white.
Tuco, very useful information and fine examples. It seems my approach is all wrong - filter use should be determined more by conditions and desired effect for each frame, rather than just picking a filter and using it for an entire roll. Obvious I suppose. As a learning experience, I'm considering shooting this roll with unfiltered, yellow, orange, and then red filters for each subject so I can see first hand how the images are affected.