Because of equipment limitations, all but my lightest beers are partial mashes. When I first began brewing, everything went into the pot at the very beginning of boil, because I believed the higher boiling temperatures of the higher-gravity solutions would give better hop utilization. However, all my brews wound up tasting and looking rather alike - strong caramel flavors with an amber color, no matter how much I backed off on the use of darker malts. There was also a sharp undertaste, a sort of mintiness that seemed to come from the hops and was present regardless of the hop varieties used.
Finally, I read here about caramelization and oxidization of the sugars during the boil. It made sense that the higher-temperature boils of a high-gravity solution would caramelize a disproportionate amount of the sugars in the wort. I found that boiling only the sparge liquor from my partial mash at near-final volumes and then adding extracts and sugars for only the last 10 minutes of the boil resulted in the complete elimination of the excessive caramel taste and a much lighter-colored beer. However the brews were still slightly darker than expected, and the sharp, minty "whang" remained.
This second off-flavor and color appears to come from prolonged contact with the boiled hops, which I had been leaving in the wort for duration of the primary fermentation. With my last few brews, I've tried to fish out as much of the spent hop flowers as possible with a strainer, which seems to have completely eliminated the second, sharp off-taste and has finally given me the colors I expect based on the ingredients used. When using hop pellets, I now achieve the same effect by racking to the secondary as soon as feasible, within two or three days.
So the lessons here would be to (1) boil at as low a gravity and high a volume as reasonable, adding only a small portion of your extract if doing an all-extract brew and (2) don't allow the fermenting wort to remain in contact with boiled, spent hops for any longer than necessary.