The background colour of Outline view note icons, or dogears in other views, changes according to the age of the note.
The colour reflects the time passed since the note's date of creation (or subsequent latest edit); it is initially light blue, "dries" after about a day to white, and then gradually yellows over the course of a year. The duration since creation/edit for triggering colour ageing transitions is not documented.
In the illustration, note 'Badges' has just been created. 'Badge pop-up picker was edited within the last day or so, 'Browse' less recently than that but more recently than 'Agent Queries', etc. 'Attribute Browser Query pop-up' shows the fully aged colour.
See also—notes linking to here: