You can use the diff language tag to generate some colored text:
- text in red
+ text in green
! text in orange
# text in gray
@@ text in purple (and bold)@@
However, it adds it as a new line starting with either - + ! # or starts and ends with @@
enter image description here
It's worth mentioning that you can add some colour in a README using a placeholder image service. For example if you wanted to provide a list of colours for reference:
#f03c15 #f03c15 #c5f015 #c5f015 #1589F0 #1589F0
Reference : How to add color to Github's README.md file
https://gist.github.com/luigiMinardi/4574708d404cdf4fe0da7ac6fe2314db