Here are the extra methods which I have in the app class:

void CMeetingScheduleAssistantApp::SetBitmapBackgroundAsMenuColour(HBITMAP hbmp)
{
    UpdateBitmapBackground(hbmp, true);
}


void CMeetingScheduleAssistantApp::SetBitmapAsGrayScale(HBITMAP hbmp)
{
    UpdateBitmapBackground(hbmp, false);
}

void CMeetingScheduleAssistantApp::UpdateBitmapBackground(HBITMAP hbmp, bool enabled, COLORREF crBackground /* GetSysColor(COLOR_MENU) */)
{
    if (!hbmp)
        return;
    HDC memdc = CreateCompatibleDC(nullptr);

    BITMAP bm;
    ::GetObject(hbmp, sizeof(bm), &bm);
    int w = bm.bmWidth;
    int h = bm.bmHeight;
    BITMAPINFO bi = { sizeof(BITMAPINFOHEADER), w, h, 1, 32, BI_RGB };

    std::vector<uint32_t> pixels(w * h);
    GetDIBits(memdc, hbmp, 0, h, &pixels[0], &bi, DIB_RGB_COLORS);

    //assume that the color at (0,0) is the background color
    uint32_t old_color = pixels[0];

    //this is the new background color
    uint32_t bk = crBackground;

    //swap RGB with BGR
    uint32_t new_color = RGB(GetBValue(bk), GetGValue(bk), GetRValue(bk));

    //define lambda functions to swap between BGR and RGB
    auto bgr_r = [](uint32_t color) { return GetBValue(color); };
    auto bgr_g = [](uint32_t color) { return GetGValue(color); };
    auto bgr_b = [](uint32_t color) { return GetRValue(color); };

    BYTE new_red = bgr_r(new_color);
    BYTE new_grn = bgr_g(new_color);
    BYTE new_blu = bgr_b(new_color);

    //change background and modify disabled bitmap
    for (auto &p : pixels)
    {
        if (p == old_color)
        {
            p = new_color;
        }
        else if (!enabled)
        {
            //blend color with background, similar to 50% alpha
            BYTE red = (bgr_r(p) + new_red) / 2;
            BYTE grn = (bgr_g(p) + new_grn) / 2;
            BYTE blu = (bgr_b(p) + new_blu) / 2;
            p = RGB(blu, grn, red); //<= BGR/RGB swap
        }
    }

    //fix corner edges
    for (int row = h - 2; row >= 1; row--)
    {
        for (int col = 1; col < w - 1; col++)
        {
            int i = row * w + col;
            if (pixels[i] != new_color)
            {
                //check the color of neighboring pixels:
                //if that pixel has background color,
                //then that pixel is the background 

                bool l = pixels[i - 1] == new_color; //left pixel is background
                bool r = pixels[i + 1] == new_color; //right  ...
                bool t = pixels[i - w] == new_color; //top    ...
                bool b = pixels[i + w] == new_color; //bottom ...

                                                     //we are on a corner pixel if:
                                                     //both left-pixel and top-pixel are background or
                                                     //both left-pixel and bottom-pixel are background or
                                                     //both right-pixel and bottom-pixel are background or
                                                     //both right-pixel and bottom-pixel are background
                if (l && t || l && b || r && t || r && b)
                {
                    //blend corner pixel with background
                    BYTE red = (bgr_r(pixels[i]) + new_red) / 2;
                    BYTE grn = (bgr_g(pixels[i]) + new_grn) / 2;
                    BYTE blu = (bgr_b(pixels[i]) + new_blu) / 2;
                    pixels[i] = RGB(blu, grn, red);//<= BGR/RGB swap
                }
            }
        }
    }

    SetDIBits(memdc, hbmp, 0, h, &pixels[0], &bi, DIB_RGB_COLORS);
    DeleteDC(memdc);
}