Which version of EA are you using?
q.
EA 12 Professional Build 1212 which was the latest build available when I went to download Build 1211.
Steps to reproduce (one of many):
1) Create a diagram
2) Add a class in the diagram (no stereotypes)
3) Use the diagram tool bar to change its fill color to something other than white (pick your favourite color)
4) Press F4
5) Click on Reset to Defaults, box on the right changes to default color
6) Press Ok, nothing happens, the class remains with a fill of your favorite color
Now try something else
1) Add another class to the diagram
2) Press F4
3) Change the fill color to you favorite color
5) Click Ok, the fill color changes (as expected)
6) Press F4 again
7) Click Reset to Defaults, color in right box changes to default

Click Ok
9) Fill color changes to default
This is a bug in my book.
Although, somehow, I am expecting somebody to give me an explanation about the differences between changing color through the toolbar and through F4.