Red LEDs are the ones which brings the smallest drop voltages, being perhaps the suited color.
Although alkaline batteries exhibit a smooth discharge profile over time, it sounds not a good idea to use a LED too close of the minimal drop voltage due to the fact that the smallest change in its characteristic would dramatically reduce the its brightness - datasheets specifies a range. In addition, depending on the production lot, and the room temperature, some displacement is expected so that you could not ensure the same intensity of the light to each component.
A voltage doubler could be made with a few amount of components.