the K stands for the COLOr
the brightness is based on the ballasts output
like if your ballasts system
all do 35W the 4300K compared to 8000K will seem less bright cuz blue will stand out more
purple is less bright than blue
but all colors other than white even green will be more bright than 4300K which is standar oem white
makes sense the K is the Kelvins color
not brightness necessarily
the brightest kits out there are german ones but phillips that cost like arm and LEG
its not available in North America
or else i woudl buy that its a 40W hid conversion
instead of 35W