You can only change the font family, size and style of most UI elements via Preferences and Settings dialogs.
Some places let you change the contrast too, but the actual colour is specified in the current skin's XML file.
You didn't say what you want to change, but if it's a tag value, you can create a virtual tag that uses a
color markup code.
e.g. To highlight tracks with no volume gain with a message in red:
$IsNull(<Track Gain>,{color: 255,0,0}"No gain",<Track Gain>)
Bee excellent to each other...