Things to think about:
Suspended yeast will change the looks of beer.
Large amounts of beer absorb light so it looks darker.
Extract is boiled at the factory to concentrate it, then boiled again when you make beer. This causes darkening due to the Mailard reaction and the longer the wort is boiled the darker it becomes.
More things to think about:
Most kits will have you either bottling after a week in the fermenter or moving the beer to secondary. Neither is of benefit. It takes time for the yeast to complete fermentation and to clean up the intermediate products of fermentation and settle out. If your yeast has begun to clump up and settle and you move it to secondary, you break up these clumps and get is suspended in the beer again and you have to wait for it to settle out again. Leave your beer in the primary fermenter for a minimum of 10 days and longer is better, especially with an imperial that will be higher in alcohol where the yeast will struggle to ferment it out completely. Don't bottle until you know that fermentation is complete by using your hydrometer and checking at least 2 samples a couple days apart. Careful racking the beer to the bottling bucket will leave most of the settled yeast behind. Letting the beer sit in the bottling bucket for a bit will let the yeast that did get transferred settle out again so it doesn't end up in your bottles.
Look at the beer when you take your hydrometer sample. In the smaller container it will look much lighter.
When you brew your next batch, only add part of the extract at the beginning of the boil, perhaps only 1/4 of it, and add the rest when you turn off the heat. That will reduce the darkening.
