Shooting with a Gray Card for Accurate Color

Its a good idea to take your first shot in a series of images with either a gray card or a Macbeth Color Checker Chart.  This way, if when you process the images you find that your Auto White Balance wasn't up to snuff and every image has a weird color cast, then you can easily correct it.

Neutral (50% gray) = 128/128/128

There's a basic online tutorial here. 
But basically, use the White Balance Eye Dropper in Camera Raw and pluck the gray card, or in Levels the center eyedropper will make any area it is clicked on neutral gray, (that is to say equal values of R, G and B). So if the color balance of a photo containing a gray card is off it can be corrected automatically by opening it in levels and clicking on the card image with the center eyedropper.

Munsell Color is a division of Macbeth. In colorimetry, the Munsell color system is a color space that specifies colors based on three color dimensions: hue, value (lightness), and chroma (color purity). It was created by Professor Albert H. Munsell in the first decade of the 20th century and adopted by the USDA as the official color system for soil research in the 1930s. Munsell Color is the international scientific reference for the colors of artifacts.