I know that they were trying to find the average wavelengths of visible light, and I loathe pedantry, but shouldn't the answer be "black"?
With a clear night and a good telescope there are a lot of stars out there. So I would say even the average color of the night sky looking out from earth wouldn't be black. http://apod.nasa.gov/apod/ap150101.html
Average Color (rgb): 62, 45, 51
Average Color (hex): #3E2D33
Average Color (visual): http://i.imgur.com/xDBeHqu.png
Source: http://pastebin.com/vckL3AP6
Credit: http://charlesmartinreid.com/wordpress/2012/08/python-image-averaging-and-color-averaging/ (with some cleanup)