Applying colour and formatting to text on a sign using the section sign (§) is broken.
You can type the section character character and everything behaves correctly but when you click done the colour and formatting is discarded.
While editing the sign:
[media]
After clicking done
[media]
§ color formatting has been deprecated for ages and is being phased out.
It not working is not a bug (the disparity between gui and final is); signs just json text to color it, or dyes.