Based on the text in the statistics page ("Show rating statistics" from the user page) the rank color is computed based on the percentile in which the user is (best from casual and competitive).
Still, there are users in the ladder that are in the first 5% of casual, and they are bright white instead of xray. For example Sortale. The user right below him at the moment is xray color. See:
this statistics pageI could imagine the rating updates only when you play but Sortale had a game 23 hours ago, so 0.4% jump due to other players seems large for that ranking. Is there any other explanation/something I am missing?