Fresh notes are a very light, pale blue. After a day or so, the note "dries" to white. Then, over the course of a year or two, it gradually ages to a pale, golden yellow.
The colors are sufficiently subtle that they don't dominate the display, but they change just enough to let you know what's new and what's old. They've been available in Tinderbox outlines all along, but in Tinderbox 2.4 they're now visible in maps.
Did I mention that Tinderbox is on sale? You can save $40 this week.