FillRect(hdc, &rect, hBrush);

/**Ouvi a correção, não a rejeiteis e sede sábios. Provérbios 8:33*/

#include <windows.h>
#include "resource.h"

LRESULT CALLBACK WndProc (HWND, UINT, WPARAM, LPARAM);

int WINAPI WinMain(HINSTANCE hI, HINSTANCE hPI, PSTR szCL, int iCmdShow) {
    static TCHAR szAppName[] = TEXT("Background");
    HWND         hwnd;
    MSG          msg;
    WNDCLASS     wndclass;

    wndclass.style         = CS_HREDRAW | CS_VREDRAW;
    wndclass.lpfnWndProc   = WndProc;
    wndclass.cbClsExtra    = 0;
    wndclass.cbWndExtra    = 0;
    wndclass.hInstance     = hI;
    wndclass.hIcon         = LoadIcon(hI, IDI_ICON);
    wndclass.hCursor       = LoadCursor(NULL, IDC_ARROW);
    wndclass.hbrBackground = GetSysColorBrush(WHITE_BRUSH);
    wndclass.lpszMenuName  = NULL;
    wndclass.lpszClassName = szAppName;
    if(!RegisterClass(&wndclass)) {
        MessageBox(NULL, TEXT("Error!"), szAppName, MB_ICONERROR);
        return 0;
    }
    hwnd = CreateWindow(
        szAppName, TEXT("Background"), WS_OVERLAPPEDWINDOW,
        CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT, CW_USEDEFAULT,
        NULL, NULL, hI, NULL
    );

    ShowWindow(hwnd, iCmdShow);
    UpdateWindow(hwnd);

    while(GetMessage(&msg, NULL, 0, 0)) {
        TranslateMessage(&msg);
        DispatchMessage (&msg);
        FreeConsole();
    }
    return msg.wParam;
}

LRESULT CALLBACK WndProc(HWND hwnd, UINT msg,WPARAM wP, LPARAM lParam) {
    static int  cxClient, cyClient;
    HBRUSH      hBrush;
    HDC         hdc;
    int         i;
    PAINTSTRUCT ps;
    RECT        rect;
    switch(msg) {
        case WM_SIZE:
            cxClient = LOWORD(lParam);
            cyClient = HIWORD(lParam);
            return 0;
        case WM_PAINT:
            hdc = BeginPaint(hwnd, &ps);
            for(i = 0; i < 65; i++) {
                rect.left   = i * cxClient / 65;
                rect.top    = 0;
                rect.right  = (i + 1) * cxClient / 65;
                rect.bottom = cyClient;
                hBrush = CreateSolidBrush(
                    RGB(
                        min(25, 4 * i),
                        min(120, 4 * i),
                        min(255, 4 * i)
                    )
                );
                FillRect(hdc, &rect, hBrush);
                DeleteObject(hBrush);
            }
            EndPaint(hwnd, &ps);
            return 0;
        case WM_DESTROY:
            PostQuitMessage(0);
            return 0;
    }
    return DefWindowProc(hwnd, msg, wP, lParam);
}

/********************Arquivo: resource.h********************/

#define IDI_ICON   1

/********************Arquivo: resource.rc********************/

#include "afxres.h"
#include "resource.h"

IDI_ICON ICON "project1.ico"