Definite article before words quality, scale, color and other singular properties

The use of the definite article is to indicate the principle aspect of the property. Using an indefinite article indicates that this is just one property of many. So, "a color of my skin is white" indicates that there are potentially colors of the skin. But by using the definite article, you indicate that this is the principle aspect of that property. This practice generalizes even if it's only possible for the property to have one value, so we say "the size of the bear" rather than "a size of the bear."