It is what you celebrate, do you celebrate failure or success? Neither, you should celebrate learning. The green areas are the areas you celebrate, and there is nothing wrong to celebrate now and then you use best practices.
Ask the team members to write down the things happened in the last iteration and put them on the flipchart. Explain to them the category is not even that important, as long as the item is on the flip chart.
Be warned, this activity works great with some teams, but some teams also have no idea what to write down when they see the Celebration Grid or where to put their sticky note.