I recently scanned about 90 old slides. Many of the Ektachrome slides had become reddish with age. The techniques I used to regain more accurate colors is described here.
I set the Photo Type to Faded or old color slide. This apparently frees the HP Color wheel to remove the overall reddish tint. I often drag the little black circle around a little and watch the result in the preview. Note the numbers -88 and 16 in the image at the right. After correcting one set of slides in the same roll, one of them looked better than the others. I copied down the 2 numbers for that slide and then rescanned 1 or 2 of others, using those numbers. The results were better the second time.
If the image has a large white area, the histogram tool with Lab units is useful. I put the cursor of the white area, and adjust the black circle in the color wheel so that the a and b components are very close to 0 - this means that the color is a shade of gray (between pure black and pure white). In the Lab color space, positive a is red, negative a is green, positive b is yellow, negative b is blue. In the sample at right, a=2 and b=-5 - a pale reddish-blue. If there is a white area that is too small to accurately locate in the HP Preview window, then I do a similar adjustment in Photoshop (see below).
If the image is underexposed, I sometimes use the Exposure tool.
I feel it is better to adjust dark areas with the HP software because
it uses the full 12-bits of color data, and Photoshop only gets 8 bits.
The same argument also applies to coarse color correction.
Photoshop is able to work with the full-size image, and I use it for fine-tuning the color. I use the Color Balance and Info box (analogous to the Color wheel and Histogram in the HP software). Usually I select a white object and Highlights (not Midtones as in the image). Getting a pleasing image is very subjective at this point.
After the Color Balance looks good, I often think the colors are too pale, so I give them a boost with the Hue/Saturation tool.