Two fully charged AA batteries give off 3.0v, but as they lose juice, they can go down to ~2.0v. For blue and white LEDs, 3.0v is the lowest forward voltage you'll find, which will result in dimming at ~3/4 battery. As far as I know, there's no alternatives or ways around this.
Different colors require different forward voltages; red, orange, and yellow can all stay lit with only ~2.0v, so they should never dim. All the other colors require ~3.0v.
If someone has any other tips or advice, please share lol, I hate this problem.