The color green

I have been trying to get Anna to learn her colors for months. It's extremly aggravating for her and me but I think we are making progress. She has known white, brown and orange for many months now. Recently we talked about the color green for hours. She pointed out everything she could find that was green. I thought for sure she had added a new color to her knowledge only to find out that 30 minutes after the green session was over she had forgotten what color it was. The weird thing was she somehow picked up on yellow during our green training so now she knows yellow. Yesterday out of nowhere she showed me some things that were pink and knew they were pink. I was so thrilled and so was she. Now if we could only learn green!