QColorDialog, Custom Color, Stored where and how to delete?

[HappyCoder]

Hello,
i'm using the QColorDialog. For testing purpose i drag&drop a color
into the custom colors. Even if i close my app, after restart the custom
color remains.

1) Where does Qt store the custom colors?
2) How to delete a custom color from QColorDialog? (pressing "Delete" doesn't work)

Thx
HappyCoder