Maybe a bit... 1991 gum card... I think their first season...

^^^
Very fitting actually. Here's Pietro's first couple of cover appearances, on X-Men #4 and #5, along with the rest of the Brotherhood and his sister Wanda, the Emerald Scarlet Witch.




This was only on the covers (and for Pietro also #6's cover). Within the stories themselves, Wanda and Pietro are shown wearing their correct costume colors (scarlet for her, green for him). 

A couple of issues later, in X-Men #7's lettercol, a reader asked "By the way, if the Scarlet Witch is scarlet, how come she's always green on the cover?"

The response from Mr. Lee/Marvel editorial: "Sometimes we change a costume's color on the cover to balance the design a bit."

I was one who voted for blue though I saw green first. And I think Pietro looks terrific on that X-men 4 cover. That may be the most chill he's ever exhibited. Everyone else is leaning in, getting their evil on, and he's just like, "Eh, whatever."

Looking at Wanda in the wrong color (and posture), there's no escaping that Green often connotes something negative, be it envy, or illness, or inexperience. Conversely, the cool blue of Pietro's later costume suggests a relaxed, untroubled serenity, that of a man comfortable in his own skin despite the occasional flash of anger or outrage. He's just passionate. 

Top Ten Things That Are Better When They're Blue


10. Steel

9. Whales

8. Man Groups

7. Clapton

6. Quicksilver's costume

5. Monday

4.


  



3.  Lagoons

2. Oyster cults

1. Suede shoes

I voted "other" because I don't really have a strong preference.

I'm a little surprised to realise that Quicksilver has had only two costumes over all these years - or were there some others that don't appear in the poll? The only one I see after a quick image search is a sort of loose-clothing variation on the light-blue look, probably from the 80s or 90s after I had stopped reading Marvel.

He's had other costumes over the years. Here's a link to a resource from the eXcellent Uncanny X-Men.net site: uncannyxmen.net/characters/quicksilver/costume-gallery.  Great site.

However I took this to be a poll specifically about his classic look, as rberman included images of such in his post. So for me at least, this poll was about  Pietro's iconic costume and whether one prefers that costume in green or blue.