This would definitely be a customization since there is no configuration parameter that defines this.
You could start by looking at the Service Desk styles sheets (i.e. analyst_styles.css, styles_ah.css)
Look at the 'requiredlabeltext' and 'requiredlookup' variables and provide the appropriate RGB #.
Copy the style sheet CSS files to the NX_ROOT\SITE\MODS folder and change to the color of your choice.
Since this is considered a customization, it is outside the scope of CA Support.
Good luck!