I did a bit of research in to this recently..
The lamps used are tiny little H6W - the same as BMW use as side lights in some of their cars. After a bit of searching around I found
these bulbs that fit and are made with CREE LEDs, so allegedly 60% brighter than normal LED bulbs.
They were a little tight to get in to the holders but with a little perseverance they went in and, despite being larger than the lamps that came out, fitted in to the light cluster without issue. Only just fitted them so don't know what they're like at night, but there's a definite colour difference visible in the daytime - it's certainly much more obvious in person than it is in the photo. Also, no bulb warnings yet, but I only had the engine going long enough to check they were working and the colour so that may change. In the photo below I've changed the offside lamp but not the nearside (apologies for the state of the car, had a trip to hospital yesterday so she's a bit dirty).