If you want the lights brighter, replace all the standard bulbs with new ones of the same spec and clean the contacts between the bulb holder and the gauge housing.
Putting the blue frangers on the bulbs will not help at all. If you want to colour the lights then I would look at wiring up some LEDS. Bare in mind that they will not work on the normal dimming circuit though. If you want them to dim you will need to power them with an adjustable voltage regulator.