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[deleted] I've seen the 16-bit color message appear when GRUB appears, so I think the OS in itself does support 32-bit color.
Perhaps, but the problem may be that it defaults to 16-bit video? That's just a guess, but I suspect that the older of the two virtual video "cards" that VirtualBox provides is able to handle 16-bit video, whereas the newer one is not. Remember that I didn't get anywhere with it until I changed to the older one.
And it was installing the guest additions, in an attempt to resolve the resizing issue, that caused things to go downhill from there.
The other possibility is that the video card in your hardware supports 16-bit color. I think that might be more likely.