Normally this color is black for most of the newspapers. Non advertising material includes news articles, editorials, opinion pages, etc. The same logic applies to magazines as well. Run of Press color is normally the color printing done during the regular press run.
Run of Press refers to the process in which the advertiser doesn’t specify a place in which his ad is to be published in the newspaper and the position is up to the newspaper’s discretion. Hence, run of press color derived its name from the same fact to refer to the color which is not given in a special and specific position in the newspaper.
One example of how run of press color was used to fight competition is as follows: In the 1980s in the US, USA Today was a national newspaper which differentiated itself with bright colored weather news which spread over a full page. Smaller newspapers tried fighting this situation by investing in color presses, but larger newspapers fought the situation by retrofitting the letter-presses that they had with run of press color.