Basically if you use hex colors in raw JSON, it will not be the same in chat. Say if you used #FF0000 as the color component, the output in chat would be instead #FB0000. Likely a similar issue to MC-209682.
Comments 3
I completely reset my chat settings by deleting any lines in options.txt that relate to chat functions and it still has this issue.
Have you changed your chat settings? (Especially transparency of the text)