That's right, the tree trunk should be brown, not green. And thus, she rejected his picture.
Grant told me that when his teacher rejected his picture, he said, "小气!"
That is to say, "I'm only six and I can see how incredibly petty you're being."
I asked him if he seriously said "小气!" to his teacher.
He said he said it in his heart.
Bless your heart, Grant. I love your picture and am forever immortalizing it on my blog.