How do I color correct using Curves?

Try to retain as much of the tone, detail, and contrast of the original. While the histogram can give you a quick picture of the tonal range of the image, adjusting the levels (i.e., by setting the highlight and shadow points at the ends of the histogram) is not a reliable substitute for tone- and color-correcting by the numbers - you lose detail because the highlights will be blown out and the shadows will be filled in. The best way to ensure a smooth histogram without "combing" is to start with a quality scan or digital shot.

To color-correct by the numbers, adjust curves on separate channels to remove unwanted casts and to move the highlight, shadows and neutrals into an acceptable range. What causes the color problem in these areas is usually also affecting the rest of the image, so use selections and selective color changes judiciously as these only address isolated areas.

Determining and adjusting highlight values
1. Use your cursor to find the lightest value indicated by the info palette. In many cases, this value is white. (The color sampler tool is a good way to hold the values temporarily while you compare values.)
2. If this value is intended to be white, use target values of RGB 240-245 to ensure the highlight details are not blown out. If the image has no white value, use your judgement to determine values. If the image has a cast, adjust the values so the cast looks clearly intentional rather than accidental or discolored.
3. Use curves on separate channels to adjust highlights so they are within the acceptable range. To improve contrast and detail in the focal area you may choose to sacrifice detail in small, insignificant areas of white. The values of specular highlights (bright, mirrorlike reflections, as on chrome or glass) should be blown out: RGB 255/255/255.

Determining and adjusting shadow values
1. Use your cursor to find the darkest value indicated by the info palette. In many cases, this value is black.
2. If this value is intended to be black, use target values of RGB 10-15 to ensure the shadow detail does not fill in. If the image has no black value, use your judgement to determine values.
3. Use curves on separate channels to adjust shadows so they are within the acceptable range. To improve contrast and detail in the focal area you may choose to sacrifice detail in small, insignificant areas of black.

Determining and adjusting neutral values
Use your cursor to find values that should appear as neutral. This will help you determine whether there is an unwanted color cast in the image. These target values have roughly equal amounts of red, green, and blue, e.g., 150/146/153. Use curves on separate channels to move neutral values within the acceptable range.

Be wary of making drastic corrections and oversaturating colors. These can create a posterized effect, and possibly move colors out of the printable color gamut. It is common knowledge that colors viewed on a monitor can not be matched identically in print - but oversaturated colors can leave customers especially disappointed with the final printed output. Use the info palette with RGB and CMYK displayed to alert you to colors that are becoming oversaturated.