Releases with end dates in the past and start dates in the future are both greyed out, so you can direct visual focus to releases that are currently in progress.
Releases currently in progress (i.e. with a start date before today's date, and an end date in the future) can be green, amber or red, and these colors correspond to the release's risk level. Check out this article for more on release risk.
An example roadmap, with three releases of different risk levels.