Differences in stock rear shock by year.

Is there any difference between the rear shocks from 2015-2017. I know in 2018 the rear shock was improved to include rebound adjustment, but from 2015-2017 there are 3 different colors silver red and blue. Is it just for aesthetics or are the springs stiffer for say the 2017. Yamaha increased the spring rate for front in 2016 and was wondering if there did for the rear as well.