If you’re using a mobile device, it should be in settings? I don’t know if it’s different for devices other than Apple, but I go into settings, click accessibility, and then find display and text size, and from that theres a color filters button that you can turn on to use greyscale.
I select random colors afterward and then use the color picker to pick on out. Although I use procreate.
It kind of depends on your device and your drawing app-