Icon tray.. do you mean the Quick Launch bar, the Task Bar or the Notification Area, or the background of the icons on the desktop? All can be what you are talking about, but you only change the background colors on the Desktop, the rest you can only change when you change your Appearance or Themes.
For Desktop, Right-click on the desktop, go to Desktop tab, you can change the full background color, and also go to Appearance, Advanced, you can change many settings there under the Item field.