NLM has a piece on the use of the colour green in the Orthodox Church as a sign of the Holy Spirit and of Pentecost. I had always delighted in the Holy Spirit in the Rublev Trinity, being clothed in green, I had assumed it to be an insight peculiar to him.
The pictures show Pentecost in an Orthodox Church.
Courtesy of Byzantines.net, here is an explanation of the difference:
"Since Pentecost was originally a feast of harvest, as was mentioned above, the Jews used to decorate their homes with the fruits of the harvest—flowers, green foliage, garlands etc.—in order to add more pomp and solemnity to their celebrations. This same custom was also adopted by the Christians. To them, however, the green branches and flowers took on a symbolical meaning—the divine life and gifts of the Holy Spirit.
There is the rather sad story of Pope Paul VI going to the sacristy to say Mass on the Monday after Pentecost, and seeing green vestments, he asked why the red vestments for the Mass of the Octave of Pentecost were not there. He was told, "But Holy Father, you signed a document abolishing it." Apparently he burst into tears. It does seem a shame we don't have an Octave now, The feast of the Holy Trinity seems to stand alone, a bit like a liturgical after thought, whereas in the past it was the Octave feast of Pentecost and the end of the cycle that began with Ash Wednesday.