Not sure if this is exclusive to the pre-release build, but there's something weird going on with Auto Color in 2.6.2 for Mac - Reaper 5.0 pre12. Hope this is the right place to post this.
As you can see in the gif, (any) + Random works as expected; the color sticks to the track when moved. However with (any) + Custom, the color sticks to the position rather than to the track, and this can be quite confusing.