A red light has been the traditional choice for retaining your night vision since before WWII when the military settled on red as the best choice. Recently, there has been a move to green and blue-green light, precipitated in large part by the military's change to green, which itself has been primarily motivated by the increased use of night vision equipment. As it turns out, green light also offers some advantages over red as a means to retain night vision capability. However, it isn't cut and dried.
Total brightness, or illumination level, of the light has a potentially more significant effect on night vision retention than does the choice of red or green. Because your eyes are more receptive to green light, we gain better visual acuity at lower light levels than when using red light. Green also allows for differentiation between colors that red does not and the magenta used on aviation charts, for example, is readily readable under green light, not always the case with red.
Both reasons contribute to the fact that pilots and many others generally seem to prefer green over red, it simply makes it easier to see and read in the dark cockpit. The potential problem is with the actual illumination levels we use, not the color of the light. The brighter the light, the more negative impact on night vision, both in our capacity to see and in how long it takes to gain back optimum night vision. This is true regardless of whether it is red or green.
Ideally, you want to use only as bright a light, red or green, as is necessary to perform your chores and no more. However, if you have a brighter light than you actually need, a brighter green light will generally have a more negative effect than an equally bright red light. Green or blue-green has a greater capacity to adversely effect night vision because the eye is about 100 times more sensitive to these colors, so even moderately too bright light can have a serious deleterious effect.
Another complication is that individuals' visual acuity at low light levels varies quite a bit, so what would be perfect for one, might be too bright or too dim for another. In other words, without some means to vary intensity, odds are no light will be perfect. Bottom line is that red or green will both perform adequately, but what you really should be more concerned about is to avoid very high illumination levels, of either color, if retaining night vision acuity is your goal.
NOTE: For more information on this subject, an excellent article is "Seeing Red" by astronomer Doug Kniffen.
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