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m_image = new Gdiplus::Bitmap( img_w, img_h );
m_graphic = Gdiplus::Graphics::FromImage( m_image );
Gdiplus::Color c( 0, 255, 0, 0 ); // ARGB = 0x00FF0000
m_graphic->Clear( c );
m_image->GetPixel( 0, 0, &c ); //ARGB = 0x00000000 ?!
The color of transparent part of the image is always black. How can I change this?
                While you can always Lock the bitmap and fill it with whatever data you want, I'm not sure what you're trying to accomplish. The RGB channels have no meaning when opacity is 0. I don't know why they're discarded, but it's not surprising.
– Esme Povirk
                Feb 15, 2019 at 4:47
The Graphics::Clear method clears a Graphicsobject to a specified color.
I have tried your code:
Image m_image(L"C:\\Users\\strives\\Desktop\\apple.jpg");
Graphics *m_graphic = Graphics::FromImage(&m_image);
Gdiplus::Color c(0, 255, 0, 0); // ARGB = 0x00FF0000
m_graphic->Clear(c);
graphics.DrawImage(&m_image, 30, 20);
delete m_graphic;
The final picture is like this.
I think the problem is clear. If you use the clear function and set the color to (0, 255, 0, 0), which defaults to black, then the printed area must be black, and the color of the pixels captured by the GetPixel function below you must be black.
        
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