something in the classroom...

I am sitting in this black holder thing. I am not alone; some of my friends are sitting here with me too. There are two green above me and one black under me(my friends). My color is blue. Not only do I have blue on me, but also white. I can give something color and make it pretty. I get used a lot during the week mostly by teachers; however, on the weekends I'm sitting here as usual with my other colorful friends.