I had this same problem! I have litronic headlights on my 2000 boxster, and when in the shop they replaced one headlight assembly. The new headlight was brighter, and whiter - while the other older headlight was not so bright and had more of a blue tint to it - which is what I liked.
I did the same thing you did, thinking it was the bulbs... but (and perhaps someone can explain this better than I can) it has something to do with the headlights age. I've had my headlights slightly unmatching for about 6 months now -- but I can already tell that they are almost normal and matching. You just have to stick it out and let the other headlight "die down" a little and they will start matching in color. They get more of a blue tint and slightly less dimm as they age. The only other option is to buy 2 new headlight assemblies - which we know is $$$$.
Thats what I found in my situation.. and thats what I heard from my local Porsche shop... good luck with that. I'd say just hold off and let mother nature match them up. I know its annoying as hell... but...