Colors are usually encoded using three or four axes, but yes, you can calculate an intermediate color by taking an average along each axis. But it's not as simple as averaging between two images. You have to make sure you're comparing corresponding body parts, regardless of posture, view angle, and size.
(TVs and Computer displays assign numeric values to Red, Green and Blue. The higher the value, the brighter the image. If you max out all three axes, you get white; all zeroes represents black.
Print images often use a system called CMYK (Cyan, Magenta, Yellow, Black). The black isn't theoretically necessary, but it's convenient for printers because black ink is cheaper than colored.
A different approach is HSV (Hue Saturation Value) : what color (coded as an angle on a color wheel), how intense, how bright.
