Reprinted with permission.
Reference:
http://www.rockhounds.com/
rocknet/archive/messages/47991.shtml

The only colors of the dyed Brazilian agates and geodes that don't eventually fade are the reds and blacks due to the processes used.

The Pinks, Greens, Blues and Purples fade with exposure to sunlight due to the chemical dyes used

The reds are created by boiling the agates in a ferric iron solution and then heating to produce red ferris iron. This does not fade and is the same method used to produce most carnelian on the market today

The blacks/dark browns are produced by boiling in sugar water and then treating with sulferic acid to convert the sugar to carbon black. Its the same method used to produce most black chalcedony and to treat the Adamooka Matrix opal.

The major problem with getting the natural geode/agates from Brazil is that the good quality agates are not as common as the pale agates which readily accept dyes. The work involved is the same but the profit is in the tourist trade so the majority of the work goes into collecting the lower grade agate for treatment rather then the few collector agates.