My almost-new desktop Wintel PC has suddenly started to display in [EDIT] greyscale [EDIT] for no obvious reason:

Hardware:

Gigabyte Z370N mini ITX mother board (I have three of these boards - no problems with the other two PCs)
Intel i5 Coffee Lake (Gen 8) CPU - not overclocked
16GB DDDR4 RAM
Samsung SSD system drive (M2)
NVIDIA GeForce GTX 3GB graphics card, with latest drivers
Windows 10 Pro 64-bit, fully updated
32" BenQ monitor (2560 x 1440)

At cold boot:

normal white-on-blue text
then normal hi-res screen saver in colour
then a hi-res [EDIT] greyscale [EDIT] screen

Testing:

swapped monitors - no fix
swapped display port leads - no fix
swapped graphics cards - no fix
system restore to 4 weeks ago - no fix
back to older graphic card software - no fix

The PC runs cool - Speccy shows a CPU temp of 29C and the graphics card is 37C

Your advice please