For me, I had a set with the 4300k and they were a little to yellow for me. Don't know if it was the brand or not, but looked just like regular halogen bulbs in them. This set, I have 5000k and to me they look pure white.
Don't know about the hardest, but definitely the most frustrating, was figuring out how to mount the shrouds on. The fact that instructions are not included really added to that. The shrouds have threaded screw holes in them, the projectors have screw holes in them, but the two holes don't line up. Contacted retrofit and they told me they mount with the black rings that were included. Even with the rings, I still couldn't figure out how they went together. Had to contact them again to find out how they go together. Really would be a big help if they just threw some instructions in on how to assemble their parts.
I didn't trust the rings holding them on with just silicone. So I drilled a couple small holes in the shrouds and used some bailing wire just to be sure everything stays together.