What normally happens with these bulbs is that as they age, they become dimmer, and the colour temperature changes towards 6000-8000k - assuming they don't go pop first.
If they are blue, then your light output will be lower, but in my experience, it is more likely the headlamp covers getting dirty inside or potted with stones that reduces their efficiency.
Some headlamp covers have a UV film that goes milky over time. Remove this and polishing the lenses will restore a fair amount of light output
