If yours is a 2008, it'll need D2S HID bulbs. The yellow crappy one will be the OEM 4300k and at some point the other bulb has went and they bought a 6000k or 8000k bulb to replace it. The higher the number the bluer it gets, until you hit around 12000 where it starts going green i think.
This is what I hope has happened, because mine was the same when i bought it, so without checking things i purchased 2 new bulbs so they'd be the same. When I went to change the drivers side over i noticed it was a completely different type of HID bulb, the type that Corsa boys illegally fit to they cars to make the lights look better. The reason it was this type of bulb I found out later was because the previous owner drove the car in snow and the splash guard fell off leaving the back of the light and ballast exposed to the elements which in turn fried the ballast. Instead of buying a new one for £60-£80 he brought it to a mechanic who did a cheap as job and fitting a Chinese HID kit.
Although not the end of the world, I had to buy a new OEM ballast and cut wires etc to get it all back to how it should have been in the first place. I hope this isn't that case for you, though