Back in the day when my beard was dark, it did have some reddish color.
Mine used to. Those hairs are now grey. Aging sucks!
You could make an analogy to tree leaves. In the summer all leaves are green because of the chlorophyll, but in the winter the actual color of the leaves themselves can show as red or yellow. Similarly all hair starts out yellow and orange and is then darkened to varying degrees of brown or black. Even blond hair has some brown or black in it.