I have pretty lame method but I use PNGquant and run my image through it several times.....there can be alot of trial and error involved depending on how obsessed you get.

https://pngquant.org/

DATimage can also reduce colours "Change -mic256 to -mic128 , mic64 etc....

DATimage pathWhereFilesAre\*.png -mtc15 -mic256