In case of metal deficient defects in the crystal lattice of a metal, the positive voids are filled by electrons.
- How does this result in the coloration of a substance?
- Does this condition arise only in the case of metals?
For a simple explanation, we could explain the coloration of the substance by the absorption of the energy of the electrons in the void, which leads to the color of the compliment color (as is suggested by http://rspa.royalsocietypublishing.org/content/204/1078/406)
However, a more rigorous explanation could be found here: http://ptp.oxfordjournals.org/content/4/2/181.full.pdf
For the second question, it is written in Ceramic Materials: Science and Engineering that "the term color center is now applied to any defect, including an impurity, that produces color in an insulator". It also notes that the original observation was made by the production of F centers, which suggests that the modern interpretation of the phenomenon applies to the defects which product color in an insulator.
I'm not sure if I answered you question exactly as you hoped, but if you look into the resources, there is a wealth of knowledge there.