Highlight object groups in keynote

I have 3 groups on a slide that I want to highlight 1 at a time. Is there a good way to do this? My slide has a gradient background so putting a rectangle over the objects and faking it mucking with their transparency doesn't seem like a good approach.

Is there a way to make objects start out opacified? If so, I can use the opacity action on the objects, but when I tried to use an opacity action, on transition, with duration 0, it didn't work. Having a short (0.1s) duration causes kind of a jarring flicker.

I also thought about using multiple slides with magic-move handling the opacity changes, but I didn't see a way to change the opacity of a group.

I'm running Keynote 6.5.2 on Yosemite (the most recent releases of both I believe).