C / C++

//Setting the background color of a window during window class registration
WNDCLASS wc = { 0 } ( or WNDCLASS wc; memset(&wc, 0, sizeof(wc)); )
...
...
...
wc.hbrBackground = CreateSolidBrush(0x000000ff); // a red window class background
...
...
RegisterClass(&wc);

// Setting the background during WM_ERASEBKGND
LRESULT CALLBACK YourWndProc(HWND hwnd, UINT umsg, WPARAM,LPARAM)
{
    switch( umsg )
    {
        case WM_ERASEBKGND:
        {
            RECT rc;
            GetClientRect(hwnd, &rc);
            SetBkColor((HDC)wParam, 0x000000ff); // red
            ExtTextOut((HDC)wParam, 0, 0, ETO_OPAQUE, &rc, 0, 0, 0);
            return 1;
        }
        // or in WM_PAINT
        case WM_PAINT:
        {
            PAINTSTRUCT ps;
            RECT rc;
            HDC hdc = BeginPaint(hwnd, &ps);
            GetClientRect(hwnd, &rc);
            SetBkColor(hdc, 0x000000ff); // red
            ExtTextOut(hdc, 0, 0, ETO_OPAQUE, &rc, 0, 0, 0);
            EndPaint(hwnd, &ps);
            break;
        }
        ...
        ...
        ...
        default:
            return DefWindowProc(...);
    }

    return 0;
}

case WM_CTLCOLORSTATIC:
{
    HDC hdcStatic = (HDC) wParam;
    SetTextColor(hdcStatic, RGB(255,255,255));
    SetBkColor(hdcStatic, RGB(0,0,0));

    if (hbrBkgnd == NULL)
    {
        hbrBkgnd = CreateSolidBrush(RGB(0,0,0));
    }

    return (INT_PTR)hbrBkgnd;
}

// ColorBkGnd.cpp : Defines the entry point for the application.
//
#include "stdafx.h"
#include "ColorBkGnd.h"
#define MAX_LOADSTRING 100

// Global Variables:
HINSTANCE hInst;  // current instance
TCHAR szTitle[MAX_LOADSTRING];  // The title bar text
TCHAR szWindowClass[MAX_LOADSTRING];  // the main window class name
COLORREF bkgndcolor;

// Forward declarations of functions included in this code module:
ATOM  MyRegisterClass(HINSTANCE hInstance);
BOOL  InitInstance(HINSTANCE, int);
LRESULT CALLBACK  WndProc(HWND, UINT, WPARAM, LPARAM);
INT_PTR CALLBACK  About(HWND, UINT, WPARAM, LPARAM);

int APIENTRY _tWinMain(HINSTANCE hInstance,
    HINSTANCE hPrevInstance,
    LPTSTR lpCmdLine,
    int nCmdShow)
{
    UNREFERENCED_PARAMETER(hPrevInstance);
    UNREFERENCED_PARAMETER(lpCmdLine);
    // TODO: Place code here.

    MSG msg;

    HACCEL hAccelTable;
    // Initialize global strings
    LoadString(hInstance, IDS_APP_TITLE, szTitle, MAX_LOADSTRING);
    LoadString(hInstance, IDC_COLORBKGND, szWindowClass, MAX_LOADSTRING);
    MyRegisterClass(hInstance);
    // Perform application initialization:
    if (!InitInstance (hInstance, nCmdShow))
    {
        return FALSE;
    }

    hAccelTable = LoadAccelerators(hInstance, MAKEINTRESOURCE(IDC_COLORBKGND));

    // Main message loop:
    while (GetMessage(&msg, NULL, 0, 0))
    {
        if (!TranslateAccelerator(msg.hwnd, hAccelTable, &msg))
        {
            TranslateMessage(&msg);
            DispatchMessage(&msg);
        }
    }
    return (int) msg.wParam;
}

//
//  FUNCTION: MyRegisterClass()
//
//  PURPOSE: Registers the window class.
//
//  COMMENTS:
//
//  This function and its usage are only necessary if you want this code
//  to be compatible with Win32 systems prior to the 'RegisterClassEx'
//  function that was added to Windows 95. It is important to call this function
//  so that the application will get 'well formed' small icons associated
//  with it.
//

ATOM MyRegisterClass(HINSTANCE hInstance)
{
    WNDCLASSEX wcex;

    wcex.cbSize = sizeof(WNDCLASSEX);
    wcex.style = CS_HREDRAW | CS_VREDRAW;
    wcex.lpfnWndProc = WndProc;
    wcex.cbClsExtra = 0;
    wcex.cbWndExtra = 0;
    wcex.hInstance = hInstance;
    wcex.hIcon = LoadIcon(hInstance, MAKEINTRESOURCE(IDI_COLORBKGND));
    wcex.hCursor = LoadCursor(NULL, IDC_ARROW);
    wcex.hbrBackground = (HBRUSH)(COLOR_WINDOW+1);
    wcex.lpszMenuName = MAKEINTRESOURCE(IDC_COLORBKGND);
    wcex.lpszClassName = szWindowClass;
    wcex.hIconSm = LoadIcon(wcex.hInstance, MAKEINTRESOURCE(IDI_SMALL));
    return RegisterClassEx(&wcex);
}

//
//  FUNCTION: InitInstance(HINSTANCE, int)
//
//  PURPOSE: Saves instance handle and creates main window
//
//  COMMENTS:
//
//  In this function, we save the instance handle in a global variable and
//  create and display the main program window.
//
BOOL InitInstance(HINSTANCE hInstance, int nCmdShow)
{
    HWND hWnd;
    hInst = hInstance; // Store instance handle in our global variable
    bkgndcolor=0x00FFFFFF;
    hWnd = CreateWindow(szWindowClass, szTitle, WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, 0, CW_USEDEFAULT, 0, NULL, NULL, hInstance, NULL);
    if (!hWnd)
    {
        return FALSE;
    }
    ShowWindow(hWnd, nCmdShow);
    UpdateWindow(hWnd);
    return TRUE;
}
void ChangeBkGnd(HWND hWnd)
{
    bkgndcolor=0x00FF00FF;
    InvalidateRect(hWnd,NULL, TRUE);
}

//
//  FUNCTION: WndProc(HWND, UINT, WPARAM, LPARAM)
//
//  PURPOSE: Processes messages for the main window.
//
//  WM_COMMAND - process the application menu
//  WM_PAINT - Paint the main window
//  WM_DESTROY - post a quit message and return
//
//
LRESULT CALLBACK WndProc(HWND hWnd, UINT message, WPARAM wParam, LPARAM lParam)
{
    int wmId, wmEvent;
    PAINTSTRUCT ps;
    HDC hdc;

    switch (message)
    {
    case WM_COMMAND:
        wmId = LOWORD(wParam);
        wmEvent = HIWORD(wParam);
        // Parse the menu selections:
        switch (wmId)
        {
        case IDM_ABOUT:
            DialogBox(hInst, MAKEINTRESOURCE(IDD_ABOUTBOX), hWnd, About);
            break;
        case IDM_EXIT:
            DestroyWindow(hWnd);
            break;
        case ID_TEST:
            ChangeBkGnd(hWnd);
        default:
            return DefWindowProc(hWnd, message, wParam, lParam);
        }
        break;
    case WM_ERASEBKGND:
        HPEN pen;
        HBRUSH brush;
        RECT rect;

        pen=CreatePen(PS_SOLID, 1, bkgndcolor);
        brush=CreateSolidBrush(bkgndcolor);
        SelectObject((HDC)wParam, pen);
        SelectObject((HDC)wParam, brush);

        GetClientRect(hWnd, &rect);

        Rectangle((HDC)wParam, rect.left, rect.top, rect.right, rect.bottom);

        break;
    case WM_PAINT:
        hdc = BeginPaint(hWnd, &ps);
        // TODO: Add any drawing code here...
        EndPaint(hWnd, &ps);
        break;
    case WM_DESTROY:
        PostQuitMessage(0);
        break;
    default:
    return DefWindowProc(hWnd, message, wParam, lParam);
    }
    return 0;
}
// Message handler for about box.
INT_PTR CALLBACK About(HWND hDlg, UINT message, WPARAM wParam, LPARAM lParam)
{
    UNREFERENCED_PARAMETER(lParam);
    switch (message)
    {
    case WM_INITDIALOG:
        return (INT_PTR)TRUE;
    case WM_COMMAND:
        if (LOWORD(wParam) == IDOK || LOWORD(wParam) == IDCANCEL)
        {
            EndDialog(hDlg, LOWORD(wParam));
            return (INT_PTR)TRUE;
        }
        break;
    }
    return (INT_PTR)FALSE;
}