Since post is done in some other video mode, it's likely the machine needs repairs.
I have an Acer One D250 that I'm running through an external monitor until I replace the broken screen (which I've disconnected).
The initial POST screen is nice bright white text. But everything thereafter is dim, including the BIOS screens and a Linux disk that I ran. I don't have a hard drive or Windows for the machine yet.
I know the external monitor is good. What would allow the POST to display correctly, but then make everything else dim?