I know nothing about building bows, so I wouldn't advise making any bets based on my comments. Just my thoughts. I can see where a slight variation in the chemicals used in getting the desired color could have an impact on the physical properties of the glass. Having said that, I doubt if it would be as big of a difference as what you would have between the properties of different types of core woods (ie. bamboo vs. maple vs. action wood etc.). It is generally accepted that there will only be a few fps difference between the different core woods when using the same limb design, as long as you use one of the standard proven woods. That leads me to believe that there wouldn't be over maybe 3-5 fps difference at the most between brown and black, or any other color for that matter. In my opinion, I could live with losing that much in order to get the look I want, whether for aesthetics or concealment or whatever. Like I said though, I'm not a chemist
Also, I'm terrible about losing return addresses, but I promise that your bow will be lovingly cared for and put to good use if that should happen. :D And if anybody has any of these with any other color glass, such as green, red, yellow, white, etc., send them as well. Might as well be thorough.
Matt Toms