I have not brewed a pure extract batch in quite a while, but you may be adding too much heat to your kettle. To minimize darkening, turn down the heat until the boil is just barely sustained. A gentle, rolling boil is all that's needed to isomerize alpha acids and drive off undesirable compounds like chlorine and DMS.
The Fiesty(sic) Goat Brewery est. 2007 & Clusterfuggle Experimental Ales est. 2009
Primary: Saison, Berliner Weisse, sLambic I, sLambic II, IPA, 70/-
Secondary: Orange Blossom Mead, Flanders Red I
Kegged: Fat Man Porter w/ 1469, Fat Man Porter w/ 005, Centennial Falcon w/ Conan, Centennial Falcon w/ Denny's, Barrel-aged Fat Man Porter, Belgian Dark Strong, Dark Mild