Why are the mountains predominately grey or dark brown?

Observing many photos of mountains one can assume that most of the mountains are grey or brown. See also the mountain article at Wikipedia.

There are however several ways a mountain can form, which allow for exposure of various rocks, still mountains are mostly grey/brown.

Why is it so? Why isn't there a single mountain out of Olivine, Pyrite, Tourmaline or, say, Autunite, or any other brightly coloured rock?

...which allow for exposure of various rocks, still mountains are mostly grey/brown.

The colourful minerals are apparent when you look up close. When you look from far away, the minerals are all mixed in your vision. My 2-year old daughter prepared a wonderful example of how this works:

Take a bunch of beautiful bright water colours, mix them together, and you end up having a greyish brown, dull colour.

Take another example. Sand, or sandstone. Boring yellow, white, or brown. But it's basically quartz. Finely divided quartz, unlike any of the beautiful quartz crystals you see for sale. Grain size is an important factor of the perception of colour.

That said, one of the skills geology undergraduates learn in their first year of university, is distinguishing colours in a geological context. What to the untrained eye may seem brown or grey, appears like a world of colour. I still remember that when I just started my geology B.Sc about 10 years ago, we were given a bunch of rocks that all looked grey. After one year, they did not look grey any more. They were red-grey, brown-grey, blue-grey, green-grey, etc.

Mountains can be coloured. Here are some pictures that I took myself a while ago:

On a closer look I'm certain you will see that there are many colours in there: green, yellow, orange, pink, red, brown, white, black. No need for geological education to see that.

Why isn't there a single mountain out of Olivine, Pyrite, Tourmaline or, say, Autunite, or any other brightly coloured rock?

Most rocks are not composed of a single mineral. However, there are rocks composed only of olivine. They're call "dunites". Here's one:

See the rock behind? It's brown. That's because olivine contains ferrous iron. Once it's exposed to atmospheric oxygen, it becomes ferric iron. A better known name for ferric iron is "rust". This is generally the case for a lot of minerals. Colours commonly arise because of transition metals in a more reduced cationic state. Higher oxidation states are usually black, grey, red, and all kinds of earthy colours. The boring colours you see are commonly the result of "rusting" of rocks.

This is also the case for pyrite. It is an iron sulfide - shiny yellow. When exposed to the atmosphere, it rusts. In fact, patches of red soil in otherwise non-red soil are an indicator for a hidden ore deposit (gossan).

As Gary Kindel in his answer correctly said, mountains may consist of either magmatic, metamorphic or sedimentary rocks or all three together.

The greyish to brownish color is thus due to a mixture of all sorts of rocks and minerals that is covered by layers of eroded material of different grain sizes. This way, mountains appear to be of a single color when observed from a distance. This is similar to mixing a couple of watercolors together - the result will either be a greyish or a brown color. However, if we would magnify the watercolor mixture under a microscope, so that we see the individual pigments, we could tell that it isn't actually grey/brown. Same applies to the mountains. From a far, they appear to be grey/brown but when you get closer you can depict all sorts of colors.

The only exceptions to this are mountains, where the material is very well stratified, so that different colors can be seen from a distance.

To answer your second question: Please note that Olivine, Pyrite and Tourmaline are not rocks but minerals.

Olivine is mostly abundant in earth's mantle (check out Moho). The chance that a mountain is build solely by a material from the mantle is highly unlikely. What happened to the material that was on top? Secondly, Olivine is not stable at earth's surface according to the Goldich dissolution series. It alters into clay minerals and iron oxides that are easily eroded by e.g. rainwater. Thus, if there was a mountain made of olivine it would have surely been eroded by now.

Tourmaline is a mineral that forms in pegmatites and is thus far too rare to form an entire mountain. Especially since there is no process that individually picks Tourmalines from nature to build a mountain out of it.

Pyrite; there is still a great debate in the literature about the formation processes of Pyrite. But it is most likely a mineral that forms in a secondary process due to the intrusion of hydrothermal mineral-rich water. Thus, Pyrite is not concentrated in one area and is, like Tourmaline, far too rare to form a mountain.

Color of mountains is due to rock composition and vegetation. Very common rock colors are grey and brown for igneous, sedimentary and metamorphic rocks. In higher elevations, predominant trees are conifers which are commonly dark greens. Also, by contrast with sky above, most mountains appear darker

There are colorful mountains though.

The Rainbow Mountains of China within the Zhangye Danxia Landform Geological Park.

Gore mountain in New York is not exclusively composed of garnet but is one of the largest garnet deposits on earth.

Graves Mountain in Georgia, USA is largerly composed of aluminum silicates minerals like kyanite, sillimanie, and Pyrophyllite.

Personally, I was amazed how much Mars and Titan looked like areas in the Mohave desert I have visited.

• Thanks for your answer, first of all. Never heard about rainbow mountains before! But please elaborate on colour-rock relation. Statement colours are grey and brown for igneous, sedimentary and metamorphic rocks is way too vague IMO as there are essentially no other rock types besides these three. Also, please clarify why those mentioned brightly coloured rocks are never found to form mountains. BTW you have mistyped one of the tags – PF4Public Aug 9 '18 at 21:02
• @PF4Public actually there are many more rock types, and continuums between these and the traditional three rock types. We just don't usually teach them to first year geology undergrads or the general undergrad because it introduces unnecessary complexity. – Gimelist Aug 23 '18 at 1:25
• Those rainbow mountains photographs seem heavily edited to show colours. In "normal" photos like those in commons.wikimedia.org/wiki/Category:Zhangye_National_Geopark those landscapes are still beautiful but in a more "rocky" colour. – Pere Jun 23 at 20:50