Happy to help!
You can turn that on by selecting
View -> Gantt Chart -> Show Group Shading
(It's a document-wide setting.)
The group shading will match whatever task bar color you choose for the group row. The task rows don't currently inherit the same color, but you can set them to be that way. Email the
support ninjas if you'd like to vote for that change in a future version of OmniPlan.