Oange document at U. S. Customs-AMS/ATL

When my mother-in-law went through customs (Dutch citizenship) last week in
ATL, they gave her an orange folder for her travel documents and sent her to
separate room. There were a lot of people in the separate room, but it did
not seem she got any special examinations ;-)

The extra attention seemd to be in response to the question, "Have you been
to the US in the last year?" She had.

Does anyone know what the color-coding means and what the criterion are for
use? Any other colors?

TIA,

James