Chalkboard art

I went to Grace's room to check on her and when she saw me, she asked, "Are you mad?" 

I replied with, "Why would I be mad?" 

She simply pointed at the wall and said, "Yook!" (Look!)

I looked at the wall and then back at Grace and said, "Honey, we're not supposed to color on the walls." 

Grace looked at the wall and then back at me and asked, "Why not?  It's a bewful pixur."  (Why not?  It's a beautiful picture.) 

What could I possibly say to that?