It is the nature of things that matters, not the colors
Are there exact colors for yin and yang?
The answer is yes and no.
Yes because we can indeed use colors to contrast yin and yang. No because there is no fixed rules as to what the color is for yin and what for yang.
In a nutshell, do not be carried away by the colors.
Yin and yang are two sides of a dualism. Yin is passive, feminine, and subdued. Yang, in contrast, is active, masculine, and expressive. You can use any color to represent yin — similarly for yang, — so long as you can bring out the contrast.
In other words, yin and yang refer to nature of things, and color is only one of the myriad manifestations of nature of things.
What about the colors for the yin yang diagram, then?
There are two colors in the yinyang diagram. Most often, they are black and white. In this specific case, yin is black and yang is white. As black in contrast to white can be gloomier and appears to be negative, ascribing black to yin fits its nature and helps to illustrate yin and yang relationship well.
Do not be dogmatic about it nevertheless. Even in the case of yinyang diagram, you can have the diagram in other colors, so long as you can bring out the contrast between yin and yang.
Do not confuse yin yang with the Five Elements — the metaphoric relationships of everything in the universe.
The Five Elements include wood, fire, earth, metal and water. They are each associated with an assortment of colors. Wood is associated with green, fire with red, earth with yellow, metal with white and water with black.
Each of the five elements, nevertheless, consists of their pair of yin and yang. For example, fire has yin-fire and yang-fire and water has yin-water and yang-water. So again there is no absolute color association, as far as yin and yang are concerned.