A bedside clock for a dark room: red, dim, and with the seconds hidden so nothing moves. Red is the conventional choice for night displays because it is less rich in the short wavelengths thought to suppress melatonin, and because a dim red readout is easy to glance at without fully waking.
Brightness matters more than colour
Colour choice is the well-known part, but overall brightness is the bigger factor. Turn the screen down as far as it stays readable, and let the device's own night shift or reduced-blue mode help. The reasoning is set out in red light, bedside clocks and sleep.
Whether to see the time at all
Sleep advice often suggests not checking the clock during the night, because doing the arithmetic on lost sleep tends to be more disruptive than the light. A dim clock you can glance at without calculating is a reasonable middle ground.