Color Corrections Using Curves
The proper means of performing image-wide color modifications is through curves -- not by using the sliders.
Modifying colors with curves is similar to modifying tones. When we speak of modifying tones it is in terms of lightening or darkening. For colors, on the other hand, we speak in terms of addition or subtraction. In addition, a modification of a color component -- red, green, or blue -- can also be expressed as an inverse adjustment of its color complement:
For example, if your image has a cyan cast you correct it by applying a curve that adds red, its complement. Or to say that an image lacks yellow is equivalent to saying that blue must be subtracted.
The common practice in finding the correct color setting is to evaluate each correction quickly because the human visual system rapidly accustoms itself to erroneous settings.
Post Color Correction
After a color change is applied, a compensating adjustment of brightness may be required if the image's tonality is to be preserved, something you may be familiar with if you've developed color prints in the darkroom. If the color correction curve was shifted upwards, which has the side effect of increasing brightness, the compensating RGB curve would be downwards, and vice versa. Image tonality, or luminosity, is especially sensitive to changes in the green channel, which has a far greater weighting than blue or red in calculating luminosity (see Color Channels and Luminosity).
Use Analog Gain to Control Exposure; Use Curves to Correct Tone and Color Balance
Analog gain is for adjusting scanner exposure, not for controlling tone or color balance. Although analog gain affects color balance, it only does so as a side effect to setting exposure, its sole function. The proper order of scanner operation is to set for optimal exposure, if necessary, through analog gain (a pre-scan operation). Optimal exposure only coincidentally results in correct balance. Then the image's tone and color balance are achieved through curves (a post-scan operation). In addition, because it operates linearly like a slider, analog gain is a generally inadequate and cumbersome tool for controlling tone and color balance. If you value your sanity, don't try to accomplish both with analog gain. Keep the tasks conceptually separate and address each with the appropriate tool.