Really this applies to a lot of the solar system worlds, just the moon is the nearest and most visible. Why is it gray? I'm thinking there are two contributing possibilities :

(1) it looks gray because human vision is limited to distinguishing only a narrow range of the spectrum; if we could see a wider range of colors it wouldn't be so gray.

(2) it really is gray, even well beyond the range of human vision. This is because it is made up of gray rock, which doesn't have interesting spectral features really at any part of the spectrum.

My inner seven-year-old is begging to know "why". A bit of googling seems to indicate that (2) is more of a correct answer than (1) -- at least the only colorful images I can find seem to be forced false-color by some processing to give extreme exaggeration to tiny color differences. Is that right? Can anyone elaborate?

Here is a really cool site that discusses the spectral properties of lunar rocks and even lets you "mix" your own lunar rock spectrum!

http://ser.sese.asu.edu/SPECTRA/index.html

Take a good close look at a gravel bar in a river bed, or a pebble beach, or similar kind of collection of rocks... now, I'm saying that based on the kind of mixed rock areas I'm most familiar with, but I realize that in some places you could look at those things and see only one kind of rock. But in many places, look at a large collection of little rocks and you'll see a wide variety of shades and colours. Red, yellow, white, black, greenish, brown and so on. But step back and the whole beach, or bar, the whole pile of little rocks, looks grey. It's a mixture. On the Moon there's less variation to begin with. There will be white minerals, black minerals, a few that are slightly different, and then maybe a few coloured glasses, a bit green or orange. But mix it all together and the effect will generally be just grey again.

Phil

A couple of years ago I asked Buzz about how it "looked" with respect to color and he said it was overall gray but to the human eye there was in fact more color variation at ground level when they were out walking around (which fits what Phil is saying). And of course we know that there are visibly green rocks in Mare Imbrium.

If I remember correctly the moon has quite a brownish tinge with a lot of variety but most of the images we see are monochrome. Here's a link to some Galileo color images of the moon that demonstrate this. It is still pretty gray but I think Phil's comment explains that.

Moon looks grey, but colors can be enhanced.
Look here: http://foto.astronomy.cz/Moon_color.htm
It's czech page, but images are international .
When you want full size images, click on "Snímek 1024x1024 pix".

Well if you look at a rainforest from the air it looks mostly boring old green.........