Lots of debate continues out my way about wearing WC colours, and similar when on the bike. I tend to agree that you really shouldn't turn up to your club race in stripes you haven't earned. You will get some abuse for it too, especially from the announcer.
On the other hand, I see no harm in buying a WC jersey, and wearing it to a race as a spectator. Every sport I can think of condones this. Most encourage it as a revenue raiser. Why not wear your hero's colours if you are there cheering for him?
Back on topic... I believe that Cippollini was the first to wear a full outfit, including helmet and gloves in his Green jersey colour scheme. This was around 10 years ago. He was famous for such flambouyant outfits as the Zebraskin one, etc. Cipollini was fined by the race organisers for failing to wear the official team sanctioned uniform, but he didn't care, and earned more in publicity than the piffling fine of maybe 200 Euros. He started the trend which continues today.