To get the display bit (colour) depth on Windows, you can use:
	
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	HWND hDesktop = GetDesktopWindow();
	HDC hdc = GetDC(hDesktop);
	int bpp = GetDeviceCaps(hdc, PLANES) * GetDeviceCaps(hdc, BITSPIXEL);
	ReleaseDC(hDesktop, hdc);

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bpp will contain 16, 32 etc.