We are ALL people of color. I have never seen a transparent person! I am the proud mom of three, soon to be four, beautiful children. I am very pale brown, my husband is from Cameroon and is rather dark brown. As a result my kids are a gorgeous cream mocha. All of them know they are brown, at least more so than Mom, and less so than Dad. We love the differences in our household. We LOVE the differences in our extended families. One side is all dark and my side is all very light.
My kids are not the only ones who notice this, of course! Kids are smart, they see the world for what it is. Because a child calls someone "brown," they are not implying anything about that person other than they are brown. It is just that simple! If someone is offended because a child refers to a friend as "brown," then we all need to look at our own hand against a piece of white paper. I showed my boys this little trick when they were 2.5 or 3. Simple way to explain that skin color classifications (i.e., white, black...) is not true to name, but it is important to see a person for who they are, skin color and all. I want my kids to see skin color, not to act as though it doesn't exist. To deny that trait of someone is to take away a piece of that person's being.
My kids are medium brown and we all love it, especially their very pale cousins who are facinated by the beauty and difference of my children's skin. And my children, too, love and notice the beauty and difference in their cousins on both sides.
My point, EMBRACE your child's innocence and nurture their attention to detail.