This site is not the best place to ask about this type of question and it probably belongs in the Off-Topic forum if you insist on posting here. This is a forum for writing plugins for Half-Life games.
A readme is typically a plain text file which cannot have color. If you're using some sort of markup language in your readme then you have to see what your parser supports. If you're using Markdown you can look
here. The VSCode Markdown preview feature (make sure your file is set as Markdown) supports syntax highlighting. If you're using GitHub, I assume that they also support it also but you can easily find that out in
GitHub's documentation.