It seems to be impossible to set the BackColor of the FindPanel. I had the same issue with the FilterPanel, the BackColor is retrieved by the currently applied Style. Here are some threads on the topic from the DevExpress Forum:
How to customize the Find Panel layout
BackColor of the FindPanel when we use LookAndFeel
Go for the first approach and use
gridView.ApplyFindFilter(string.Empty);
when needed.