how can we add special caracters or colors to Custom field's values

[El Attabi Mounaim]

How can we add some special characters to our custom Fields Values i mean something similar to this:

[Mikael Sandberg]

You would need an app to do it. If you are using Jira Server have a look at Icons for Jira, Color custom fields, Rainbow just to name a few. If you go to the Marketplace you can find more.

[Nic Brough -Adaptavist-]

For the background info, the priority icons are additional functionality for priorities.  The off-the-shelf custom fields don't have anything that can do it.  So you'd need the add-ons Mikael mentions.