Yes, neon tetras lose color at night. Neon tetras typically lose their color at night or when they spend a lot of time in the dark. Their color usually fades when they are resting, and it can take only a matter of seconds or even hours.
In addition, it is also perfectly normal if not all of your neon tetras lose their color at night. Their color loss depends on their different levels of rest and varied times for rest. Also, the degree of color loss depends on the individual neon tetra. If your neon tetras lose their color in the daytime, they are most likely ill or stressed.