“He ranges the mountains as his pasture, and he searches after every green thing.” (Job 39:8)
It is not only the Lord who searches “after every green thing.” In Genesis 1, God gave every green leaf to be food for humans and for animals alike.
“And to every beast of the earth, and to every bird of the air, and to everything that creeps on the earth, everything that has the breath of life, I have given every green plant for food.” (Genesis 1:30)
Green, the color of chlorophyll, is the color of life for plants and the life giving process of photosynthesis. Maybe it is life giving and sustaining qualities associated with green that causes God as Creator to seek our and value things green.
He who trusts in his riches will wither, but the righteous will flourish like a green leaf. (Proverbs 11:28)
Green is not the color of money in the Bible, but the righteous will flourish like the well watered green leaf. I am amazed when walking in the woods about all the shades of green present in any one small portion of land. The shapes, sizes, contours of the leaves are abundantly varied. Even though the shades of the color vary so greatly, yet everyone of them is still green. Not all greens are identical.
“Blessed is the man who trusts in the LORD, whose trust is the LORD. He is like a tree planted by water, that sends out its roots by the stream, and does not fear when heat comes, for its leaves remain green, and is not anxious in the year of drought, for it does not cease to bear fruit.” (Jeremiah 17:7-8)
I do not know when or why green became the color identified with Pentecost in Orthodoxy, but it is the color of abundant life in the plant world. Traditionally in Orthodoxy the only mention of color with a feast was whether vestments should be bright or dark, but an exact color was not assigned to a feast, so I can only guess that the use of green with Pentecost must be a recent practice. I also don’t know when or why churches began decorating with tree branches and green leaves for Pentecost. It is possible that this too is a relatively recent practice.