Race in the frequency of use of words in our literature

I used Google Ngram Viewer to trace the frequency of use of the terms "Negro", "African American" and "Black" in its huge collection of books. You can see the results above. The term "African American" has been growing in use since 1980, but is still not frequently seen in books. The term "Black" has risen greatly in frequency, replacing "Negro" since 1960. I was concerned by the confusion of the word "Black" referring to an African American and the word "black" referring to the color per se. I therefore ran a search on various color words, as shown below.

The word's "black" and "red" have very similar frequencies until the 1960's when the word "black" takes off. I suppose the interpretation is that "black" referred to the color before the 1960's, and then came into use additionally to refer to "black people". Note that the word "white" is used more frequently than the words "blue", "red", "green" or "yellow", and shows changes in frequency after 1960 that are like those of the word "black". It is kind of interesting that there seems to have been an increasing discussion of colors in the literature from 1840 to 1900, a plateau from1900 to the end of World War II, and then a decrease in more recent times. Why???

I did a third plot on the words "slave" and "segregation". The Civil War shows up as a peak in the use of the word "slave", and "segregation" rises in frequency from 1900 to 1960, from which time it remains relatively constant. Actually, there seems to have been (as might be expected) an increase in discussion of slavery and segregation in the 1960s and a decrease in the "me" 80s.

The words we write do seem to reflect the key trends in history!