It's pretty complicated, I sometimes mathematically work it out via Photoshop to PES or PES to photoshop which is long sometimes, or kindly sometimes other kit makers share colours with me but most of the time I luckily stumble upon the colour by making it In photoshop and seeing if it works in the game, It can take alot of trial and error and an extremely long time, I have had times where finding the right colour has taken me longer than making the PNG but once i find the colour I store it In my colour library which saves me time when making other kits with similar colours. I don't mind at all sharing colours with others at the end of the day it is only a colour and by sharing it will help better the community.
Hope that has answered your question.