Dave E. over on the Brickset forums has compiled a fascinating summary of the evolution of the LEGO palette over the past 40 years. Dave wrote an program to analyze the Brickset database, pulling part inventories for the last 40 years’ worth of sets. He says he ignored a few special themes known for their rampant use of unusual colors, such as Duplo and Fabuland.
This chart compiles the colors as a percentage of the total parts produced each year, so while a color’s percentage may decrease from one year to the next, its actual quantity produced may increase if LEGO manufactures more total pieces the next year. This chart also only accounts for a set’s release year, and not the subsequent years in which that set may have been produced, nor the quantities LEGO produces, so it only approximates what a collector would have if they were able to buy one copy of each set in its release year.