Preferences User Interface Colors
Here you can change the colors used for some specially displayed text.
You will first select the item to change using the drop-down box and then set the color you would like. The sample texts on the right will immediately change it's color according to your settings.
this will not affect the coloring of the packet lists items. To change this, use the View->Coloring Rules menu item, for details see the Packet Colorization chapter in the Wireshark User's Guide for details.