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The Mystic Forge can be used for a lot more than just randomly combining gear and gambling on getting something more valuable while you level. You can actually combine four dyes into a single rare one for example. There's no guarantee that you'll get a valuable dye, or a particular one you happen to be hoping for, but it definitely will be an option for attempting to acquire rare dyes.
Rare dyes can also be crafted, and this is yet another example of how Guild Wars 2 enables players to invest either time or money to obtain what they are looking for.
It's going to take some time, but eventually we'll have enough data on the forge to know the math behind how it works. At that point, we can do some number crunching and determine the cheapest way to obtain and flip rare dyes.
Regardless of how you get rare dyes, it's important to know of the market instead of overlooking dyes for things like bags, weapon, or armor.