It's color coding for competitions, on high quality plates the code goes -
50kg/110lbs = dark green
25kg/55lbs = red
20kg/44lbs = blue
15kg/33lbs = yellow
10kg/22lbs = green
Large Bumper plates are all the same size to distribute the force, so color coding makes sense. Also with powerlifting plates, the 50kg, 25kg, and 20kg are similarly sized, so it makes sense there too.
It's more or less standard now at bigger international meets. 50kg iron plates are still used at some power meets, but aren't used in olympic lifting anymore because the loaders and lifters didn't like them. The U.S. had a slight different color code for many years where 100lb plates were gold colored.
On cheaper non standard US bumper sets, the color code sometimes goes red 45, blue 35, yellow 25.