I'm not exactly sure what you wish to change. Are you talking about the color of the message content, or the thread title?
The former you can accomplish with BBCode (make sure the BBC extension is enabled on your Flarum installation) by using the following syntax:
[color=COLORNAME/HEX]Colored text here[/color]
where COLORNAME/HEX is either the XHTML colorname or the hex number (#000000 for example).
The latter requires adding new CSS rules to the Appearance section of your Flarum admin panel. Please let me know if this is what you're attempting and we can go from there. Or you can try to figure it out using the Web Developer console in your browser (usually activated by F12) and find the CSS class name you need to adjust.
