Just Open the MSDN and type "Brush AND disable" on the search tab and the first result talk about it.

Extracted from MSDN technical article named: "Introduction to Graphics Programming Using WFC":

"...Logical Brush Types
Logical brushes come in three varieties: solid, pattern, and hatched.

A solid brush consists of a color or pattern defined by some element of the Windows user interface (for example, you can paint a shape with the color and pattern conventionally used by Windows to display disabled buttons). ..."