Google Ngrams is a curious little service that Google offers. It's like Google Trends or Google Insights, but instead of looking at the frequency that words appear (or rather are searched for) on the internet, it somehow calculates the frequency of words' appearing in books over the decades.
Who knows how they do it? Google is omniscient.
Here are a few NGrams. I'll add more every now and then. Remember they'll be more readily visible if you click on them.
While this is just me playing with the Google Ngrams legend (blue is blue, red is red, etc...haw haw), the results are still interesting. Red is apparently more noteworthy than green or blue, which are neck-and-neck, and yellow is a distant last. But odd that from 1900 to 1945, people spoke about the colours in about the same volumes. Then, however, there was a post-war dip lasting half a century until 1995, after which it became cool again to write about colours.
This one is quite artistic. The Cold War, the great battle between political philosophies, waged in books. Communism slowly rises, peaking in about 1965, before fading away - which more or less reflects reality. Capitalism, though, is more interesting: a double-humped camel peaking at first in 1935 before dropping almost to the level of communism in about 1955 before rising again and blowing communism completely away in the 80s and 90s. In this century, people don't care much about capitalism either.
Lastly, the Beatles. This graph goes back to 1960, when there must have been someone else out there named George Harrison - that anomaly skews the numbers a bit, but by the 21st century the four Beatles are written about in the exact order that you'd presume they were: John, Paul, George and Ringo. What's interesting, though, is just how far ahead of the pack Lennon is and how McCartney spends the 80s and 90s merely neck-and-neck with Harrison. And poor Ringo. No-one ever writes about Ringo.