Contrast, brightness, exposure, clarity, sharpening all work. And, you can do all or the normal local adjustments you would do in LR or Photoshop.
Imagine if you had scanned a b/w film negative and proceeded to work on it in Photoshop. Pretty much the same process.
A yellow or, more dramatically, orange or red, filter will enhance the contrast between clouds and sky by darkening the blue sky. A deep green filter will also darken the sky, and additionally lighten green foliage, making it stand out against the sky. A blue filter mimics the effect of older orthochromatic film, or even older film sensitive only to blue light, rendering blue as light and red and green as dark, showing blue skies as overcast with no contrast between sky and clouds, darkening blond hair, making blue eyes nearly white and red lips nearly black.
Not sure why a red filter is "horrible". I have used red and even dark red to increase contrast for certain images and results. In fact, if you look at the images on my website, the majority of the film images were shot with either light red or red filtration. The digital conversions were almost all done with varying levels of orange or red filtration.
Edited by SpiritShooter, 31 August 2012 - 18:02.