Working through a project in FCPX, I found a workaround that lets me use transitions between clips that are not on the primary storyline.

It's actually pretty simple. The trick is to place the clips you want to transition into their own storyline. If you have connected clips by themselves, you can't transition between them. Select the clips you want, right-click, and choose Create Storyline. You'll see a gray outline appear around the clips. Now, you can apply transitions all you want.

This is really useful as a workaround for color correction transitions and for audio transitions if you don't want to use keyframes.
AuthorSheffield Leithart