There are different approaches,
this is just my approach, also please do your research.
you can use a color picker software and know the code of the color and in regards you can change the color by its color code number on the color panel. (find more about this on line)
another approach is go to this link " https://www.interworks.com/blog/rrouse/2016/05/10/new-improved-color-tool-tableau " hope this helps you.