That's how MCGA works. It only has 64K VRAM, so 640x480 will only produce 1-bit monochrome. That's MCGA for you. That's why I modded my model 25 with VGA, so I could use 16 colors (or shades of grey in my case) in 640x480. (Except I later realized I installed a video controller that did 32k colors in 640x480 mode, which is amazing to load photos on).