Why can't bears be pink?

Or frogs blue?  Why can't I draw a self portrait with green hair?  I have an imagination, and in my imagination maybe I see pink bears, blue frogs, and myself with green hair. 

I work in an elementary school and I have to bite my tongue when I hear "Dogs can't be purple." or "Is grass really red?"  I think to myself "Why can't it be?  Why does it have to one way or the other to be right? "Why are we trying to stifle the imagination of young people?"  As a creative person, I think I tend to see the world a little differently.  I like to take out my imagination and play with it, stretch it, bend it, wrap it around me.  It's what inspires me to create the things I do.  Without imagination the world be a pretty dull and boring place.  Think of the movies and books that wouldn't exist if their creators hadn't used their imaginations.  No Wizard of Oz, no Harry Potter.  Picasso and Warhol would of been considered no talent hacks.  With out imagination, we wouldn't have inventors, no innovations in technology.  The world would be black and white, because it's our imaginations that make it colorful. 

So I'll ask again, why can't bears be pink?