A couple things to try. For one, you may want to consider stepping up your boil capacity to a full 5 gallon boil. The less concentrated the wort is during the boil, the lighter color wort you will produce.
There is a solution for partial boil brewers, referred to as the Late Extract method. Withhold 50% for the last 15 minutes of the boil. This will give you a lighter colored wort, as the wort is less concentrated for a majority of the boil duration.
As a benefit, you will actually get better hop utilization as well, so this method is especially beneficial for light-colored, hoppy ales and lagers.