Hi Alina,
We dealt with this a few months ago (and there is actually a post about it from mid-June (the 14th maybe?) called "talking to kids about race" if you feel like going back and reading the 22 wonderful responses I rec'd.
I am white and my husband is black. My son was noticing the difference in our skin colors. One day he said "mommy, is daddy's skin black?" I was speechless. Sure, it was something I knew that we would need to talk about, but I didn't know what to say. I actually got out crayons and showed him that daddy really wasn't black (he's more brown) and I wasn't really white (more tanish-pink) and when God mixed our two colors together, it made his skin color as well as his brother's. I was very clear to point out that there are all sorts of different skin colors in the world and that we are all a different shade of beautiful. God made all of us different, but we are all the same on the inside. Some people have short hair, some people have long have straight/curly hair. Tall/short . . .you name it, but we all are the same on the inside.
Don't make a huge deal about it. Color doesn't mean the same to kids, they are just making observations. Use it as a teaching experience.
Good Luck,
B.