It is an odd way of coloring the map. In the 1970 map, the dark yellow represents 10 to 18/100 (or the national average + or - 36%). In in the 2000 map, it represents 20 to 28, or the national average + or - 17%).
In 1970, the palest color represented areas with less than 1% of the population graduated (9% of the national average). In 2000, it represents areas with less than 14% graduates (58% of the national average). In 1970 if you were at 64% of the national average, you were dark yellow. In 2000 if you were at 58% you would be two shades lighter.
