I believe the difference in springs is how strong the spring is. Green is for your standard car. Yellow is for a mapped one.
EDIT: just read apparently green is good usually but people change them for yellow if they start dropping boost (spring being too soft to hold the piston closed)
Yep, that's right! I bought the DV off ebay a few months back. It was supposed to have a yellow spring included, but when I serviced it before installing I found it was very much green!!!!
Didn't really have any problems with the green as such, just had a bit of what sounded like surging once or twice in 4th onwards from low rpm and I read changing the spring to yellow solves it. Not noticed it since so it seems to have worked.
TBH I very rarely boot it under 3k rpm as I like to give the turbo enough time to spool up ...