We need our resident Tree Marker/Forester person to answer that. SLOWP?
'Til she gets here...some times trees are marked to cut, other times they're marked to leave. The FS uses blue a lot to mark the cut. They also use colored ribbon to mark unit boundaries and corridors. The ribbon is great because it can be removed or relocated by the logger. Not that that ever happens.
The outfit I do most of my work for usually marks leave trees and I think they use whatever color on sale at WalMart that week. We mostly clear-cut, so the only marked trees are arch sites, buffer zones along waterways, and boundary identification.
If you want to see a USFS person get all agitated...ask to borrow their blue paint for a while.