I use 3 sets of hids 35w for the low beams and fogs and 55w for the brights, I have about 4 different colors which i change occasionally depending on how im feeling lol. All of my ballasts are digital slim ballasts makes them easier to install in tight places and they look better. look for a company that has slim ballasts that are digital most new ones are digital. there is so many company's out there i have a combination of stuff from about four different company's. the difference between the 55w and the 35w is brightness and power output.ex: if you had 55w 8k the color would look the same with a 35w 6k. the 55w is brighter but you loose some color. also when using a 55w system 9 time out of 10 you are going to need a relay cause they need more power for initial start up so one might flicker then turn off indicating there is not enough power. relay fixes that. on my 55 w ones i use them for my brights with no relay sometimes they work and sometimes one doesn't work, im just to lazy to get a relay.
companies to check:
ddmtuning(lifetime warranty)
hidcountry
kryptonbulbs
hidgeek