The red and black colors do not signify anything relevant to this demonstration. The federal government releases partial updates to the CFR on a quarterly basis and changes the color from one year to the next.
Book stacks for 1950, 1970, and 1990 are represented using the average size volume in 2013, which is roughly 750 pages long. Stack size is calculated by dividing the page count in those years by 750 pages. The data for page counts in the CFR comes from here.