I looked online but what I found are only ways to disable driver signature verification on Windows 8, 10 to be able to use unsigned drivers. This is not what I need.
In my case (Windows 10 64 bit) I installed unsigned drivers (Extra Magic for Apple Touchpad) and I got the "test mode" text on the lower right. The boot/reboot/shutdown background was always red.
Now I removed the drivers and I also used Autoruns (How to find drivers breaking digital signature enforcement in Windows 7 64bit) to verify that the drivers are actually removed. In fact, something was left and loaded from System32/Drivers and it appeared yellow.
Now that in Autoruns I see no more drivers for the Touchpad (but still something else red, like the driver for the X-Rite i1 Display Pro, the screen calibrator), the boot/reboot/shudown background color is always green. This makes me think Windows too knows that no problematic drivers are present.
Now I would like to keep the check for driver signature on, as it was before, but I would also like to get Windows back in the standard mode I was used to, without "Test mode, Windows 10 build 10240" and the fixed green boot/reboot/shutdown background (before all this, the color was variable).