Backcolor on document windows

Xojo 2015
I am trying to set the hasbackcolor and backcolor properties of a document window at runtime.

If I set these in the IDE, they work: I can get for example a pink or black window
If hasbackcolor is false in the IDE, setting it to true in a constructor or in activate event, does not change the backcolor

(The IDE suggests that thee properties are obsolete, but doesnt offer customcolor in autocomplete)

Using the same technqieues DOES cause a color change on modal windows and other types
Whats special about a document window?