The check marks pop up on recommendations and statistics pages. So far, members have been particularly happy to see them on your Series Statistics page, which lists all the series in your library, and which books in the series you have in the various collections.
As should become immediately obvious, colors can’t do justice to all the complexity of collections. Not every collection gets a color, and we aren’t mixing up paint buckets when a book belongs to two collections. We may revisit this, allowing members to set colors for their collections, but it will depend on other priorities, and we’ll never be able to squeeze all the information in collections into colors alone. For what they are, however, we hope they’re useful.