How to repaint a push button in Win32?

Programmer Question

I have a window with a button that paints the entire window red when clicked, but my code paints the whole window, even the button. When I resize the window or cause the part of the window where the button is to repaint, the button will return. I've only been able to paint the button a solid color after painting the window, but that only makes it look like a square with nothing in it.

So how do I paint the button its default color?

//WinMain 
hmain = CreateWindowEx (0, L"Window", L"Window", WS_OVERLAPPEDWINDOW, 0, 0, 200, 200,
    NULL, NULL, hinstance, NULL);

HWND hwnd2 = CreateWindowEx (0, L"Button", L"Red", WS_CHILD | BS_PUSHBUTTON, 0, 0, 50, 50,
    hmain, (HMENU) redbtt, hinstance, NULL);
//Winmain

//WndProc
COLORREF red = RGB (255, 0, 0);
HBRUSH redbrush = CreateSolidBrush (red);

switch (msg) {
    case WM_COMMAND: {
        if (LOWORD (wparam) == redbtt) {
            color = 'r';
            RECT rect;
            GetClientRect (hmain, &rect);
            HDC hdc = GetDC (hmain);
            FillRect (hdc, &rect, redbrush);
            ReleaseDC (hmain, hdc);

            /*GetClientRect ((HWND) lparam, &rect);
            hdc = GetDC ((HWND) lparam);
            COLORREF color = GetBkColor (hdc);
            HBRUSH brush = CreateSolidBrush (color);
            FillRect (hdc, &rect, brush);
            ReleaseDC ((HWND) lparam, hdc);*/
        }

        break;
                     }
    case WM_PAINT: {
        HBRUSH brush;

        if (color == ' ') {
            brush = (HBRUSH) (COLOR_WINDOW + 1);
        } else if (color == 'r') {
            brush = redbrush;
        }

        PAINTSTRUCT ps;
        HDC hdc = BeginPaint (hmain, &ps);
        FillRect (hdc, &ps.rcPaint, brush);
        EndPaint (hmain, &ps);
        break;
                   }
//WndProc

Find the answer here