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. 
The color of the glow around critical tasks. You can turn the glow on and off from the View menu or the toolbar. 
The color of all dependencies between tasks.