OK....you could get white, silver or black in 2004. In 2005 you could have white, silver or yellow. In 2006 you could have white, white or white
2004 was the first year of the shock
2005: same exact shock and spring, just came tuned differently from the factory with the adjustments. Valving was all the same.
2006: New spring and valving, along with different adjuster settings.
I dont know that there really is an accurate way of telling the difference.....except that a set of 04's are going to be more used...haha.
I have an 04 yfz sitting right next to my 06 700 with 06 yfz shocks. They look identical.