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: Brett Saison, Brett Berliner Weisse, sLambic I, sLambic II, Pils, Dunkel, Oud Bruin
Secondary: Flanders Red I, Cab Merlot
Kegged: Porter, Scottish 80, Pils, Orange Blossom Mead