For as long as I can remember, I have been told to use red lights at night to protect my night vision. This has been used by the military since before WW2. Recently, the US military has changed from red to green. Part of the reason for the militaries change is the use of night vision equipment. However, as it turns out, green light or blue-green offers some advantages over red as a means to retain night vision capability.
The biggest factor in protecting your night vision is the brightness, or illumination level of the light. This has a more significant effect on night vision retention than does the choice of red or green. But it turns out that your eyes are more receptive to green light than red. Because of this, we gain better visual acuity at lower light levels of brightness than when using red light. Green lights also allow you to better tell the differences between colors than red lights. The magenta used on aviation charts, for example, is readily readable under green light, but not always with red.
Another complicating factor is variations in visual acuity at low light levels, so what would be perfect for one, might be too bright or too dim for another. Chances are that without some means to vary intensity of the light, no light will be perfect. Red, green or blue-green lights will both help to protect your night vision. But the biggest concern is avoiding very high illumination levels, of either color, if retaining night vision acuity is your goal
So when moving in the dark, you want the lowest level of brightness possible. In this case, you might want a green or blue-green light source since your eyes are more receptive to these colors than red. Red requires a high level of brightness.
You can find flashlight filters on the internet or improvise them out of cellophane paper.