Color window in hbrBackground

System Colors defined in Winuser.h

#define CTLCOLOR_MSGBOX 0

#define CTLCOLOR_EDIT 1

#define CTLCOLOR_LISTBOX 2

#define CTLCOLOR_BTN 3

#define CTLCOLOR_DLG 4

#define CTLCOLOR_SCROLLBAR 5

#define CTLCOLOR_STATIC 6

#define CTLCOLOR_MAX 7

#define COLOR_SCROLLBAR 0

#define COLOR_BACKGROUND 1

#define COLOR_ACTIVECAPTION 2

#define COLOR_INACTIVECAPTION 3

#define COLOR_MENU 4

#define COLOR_WINDOW 5

#define COLOR_WINDOWFRAME 6

#define COLOR_MENUTEXT 7

#define COLOR_WINDOWTEXT 8

#define COLOR_CAPTIONTEXT 9

#define COLOR_ACTIVEBORDER 10

#define COLOR_INACTIVEBORDER 11

#define COLOR_APPWORKSPACE 12

#define COLOR_HIGHLIGHT 13

#define COLOR_HIGHLIGHTTEXT 14

#define COLOR_BTNFACE 15

#define COLOR_BTNSHADOW 16

#define COLOR_GRAYTEXT 17

#define COLOR_BTNTEXT 18

#define COLOR_INACTIVECAPTIONTEXT 19

#define COLOR_BTNHIGHLIGHT 20

#if(WINVER >= 0x0400)

#define COLOR_3DDKSHADOW 21

#define COLOR_3DLIGHT 22

#define COLOR_INFOTEXT 23

#define COLOR_INFOBK 24

#endif /* WINVER >= 0x0400 */

#if(WINVER >= 0x0500)

#define COLOR_HOTLIGHT 26

#define COLOR_GRADIENTACTIVECAPTION 27

#define COLOR_GRADIENTINACTIVECAPTION 28

#if(WINVER >= 0x0501)

#define COLOR_MENUHILIGHT 29

#define COLOR_MENUBAR 30

#endif /* WINVER >= 0x0501 */

#endif /* WINVER >= 0x0500 */

#if(WINVER >= 0x0400)

#define COLOR_DESKTOP COLOR_BACKGROUND

#define COLOR_3DFACE COLOR_BTNFACE

#define COLOR_3DSHADOW COLOR_BTNSHADOW

#define COLOR_3DHIGHLIGHT COLOR_BTNHIGHLIGHT

#define COLOR_3DHILIGHT COLOR_BTNHIGHLIGHT

#define COLOR_BTNHILIGHT COLOR_BTNHIGHLIGHT

#endif /* WINVER >= 0x0400 */

So, as @CodyGray defined, 1 is added to differentiate between COLOR_SCROLLBAR and a NULL HBRUSH. And, (Quoting @CodyGray's comment and corecting my solution) "HBRUSH is a C-style cast and you should be using static_cast<HBRUSH> instead of a C-style cast."

Don't you mean?

wc.hbrBackground = (HBRUSH)(COLOR_WINDOW + 1);

GetStockObject gets one of the stock brushes, pens, fonts or palettes. You should not be using COLOR_WINDOW with it.

Use one of the stock brushes with it, so for a white background you could use...

wc.hbrBackground = (HBRUSH)GetStockObject(WHITE_BRUSH);

Whatever works, use it!