Just wanted to document this here for posterity. I have a 386DX-33 machine with a CGA card in it, and was getting "Display switch not proper" at boot up every time, although the machine worked fine. With a VGA card I didn't get that error. Makes sense because the VGA has its own BIOS but the CGA does not. Anyway the motherboard has no markings, nothing useful on the silkscreen, no branding etc. It's a generic TOPCAT motherboard with AMI BIOS, based around the VLSI 82C330/331/332 chipset. I had no idea which jumper was the colour/monochrome jumper. But I realised that the colour/mono setting comes in via the 8042 keyboard controller. Looking at a datasheet for a Winbond 8042 clone, it looks like it's pin 33:

I didn't peel off the sticker on my keyboard controller but sure enough next to it there's a jumper:

I traced it with my multimeter and it's connected to pin 33 and ground. Putting a jumper on it solved my boot up error!