I think I found the answer in the FAR/AIM "Everything you wanted to know about flying, if only you could find it."

http://www.faa.gov/air_traffic/publications/atpubs/aim/Chap5/aim0503.html

Section 5.3.4 Airways and Route Systems 1 (a):

NOTE- Segments of VOR airways in Alaska and North Carolina (V56, V290) are based on L/MF navigation aids and charted in brown instead of blue on en route charts.

2(b)

(b) The L/MF airways (colored airways) are predicated solely on L/MF navigation aids and are depicted in brown on aeronautical charts and are identified by color name and number (e.g., Amber One). Green and Red airways are plotted east and west. Amber and Blue airways are plotted north and south.

NOTE- Except for G13 in North Carolina, the colored airway system exists only in the state of Alaska. All other such airways formerly so designated in the conterminous U.S. have been rescinded.

So what in the heck is L/MF navigation? L/MF stands for Low to Medium Frequency and it is used for NDB airways.

From Aviator's Guide to Navigation

The third type of NDB is the en route airway beacon. These are medium-powered transmitters with a standard range of 50 nautical miles. En route airway beacons are quite common in other parts of the world, including Canada and the Caribean. NDB airways are called L/MF airways (for low/medium frequency), and are identified, like all international airways, by letter and number. Alpha and bravo airways run generally north and south, romeo and golf airways generally run east and west. (Tranditionally, international airways have been identified by color: amber, blue, red, and green.)

Pretty awesome, I'm really curious about flying one now. I think the blog Cockpit Conversations had a story about using an NDB airway for a commercial flight. NDB navigation is becoming a dying art.