Customizing Gantt chart colors with the Project Colors inspector
The background color of the whole document. This also affects the "Whole Document" background color in the Project Styles inspector. 
A color for every other item; set this to a nice low-opacity color to keep your rows distinctive. 
The color for non-working hours. But only if you've set up the View Options to show off-hours. 
Today's date in the chart. But only if you have a specific start date set in the Project Information inspector. 
The base color for lines between time spans. They fade in and out as you zoom.