One of the most important characteristics of an image is the color balance - making neutral colors look neutral and making other colors look lifelike. VueScan's default setting for "Color|Color balance" ("White balance") is able to automatically do this for the vast majority of scans.
Sometimes VueScan's white balance algorithm doesn't cope well with unusual lighting. For instance, the light from a sunset will be adjusted so that the ground appears gray instead of orange and scans of flowers will appear less intense than they should.
The best approach to solving this is to change "Color|Color balance" from "White balance" to "Neutral". In addition, if you're scanning slides, make sure you've set "Input|Media type" to "Image".
When using the "Manual" setting, it's possible to manually set the value of the neutral color. This is somewhat counter-intuitive, since decreasing the red neutral color (for instance) increases the overall red color of the image.
It's much simpler to use the right mouse button (with control key on Mac OS) to click on part of an image that contains a neutral (i.e. shade of gray) color. This will update the color balance of the image so that this point is a shade of gray. You can reset the color balance to "White balance" by double-clicking on the image with the right mouse button (with control key on Mac OS).
On Mac OS, use the control key along with the mouse button to set the neutral color (single-click) or reset the color balance to "White balance" (double-click).