Recently I was preparing to lead a Bible study on Mark 6:30-56, including the miracle of feeding the five thousand and also Jesus walking on water. Especially because we are told that "they did not understand about the loaves", I thought that there must be something more to the significance of the feeding of the five thousand than simple a display of power, another kind of miracle to add to Jesus' repertoire. There must be some common theme or thread that runs through the whole text, linking the accounts together.
With the help of some commentators, I saw that Jesus' compassion toward the people because "they were like sheep without a shepherd" (Mark 6:34) was not coincidental to the following miracle that was performed. This same expression (more or less) is used a number of times in the Old Testament, eg in these passages: Numbers 27:12-23; 1 Kings 22:17, Ezekiel 34:1-31; Zechariah 10:1-12. There is the emphasis that the people needed a shepherd, sometimes that shepherd is a human leader (eg Moses or Joshua or Davidic king), and other times the LORD himself is their shepherd.
And what is one of the significant tasks of the shepherd, if not to feed his sheep (cf Ezekiel 34:2, 34:3, 34:8, 34:13, 34:14, 34:23, etc, also John 21:15-17)? And here the green grass comes in. In the midst of a miracle of Jesus feeding the people, Mark records for us that Jesus "commanded them all to sit down in groups on the green grass" (Mark 6:39). Although the commentators I read didn't mention it, I couldn't help but be reminded of Psalm 23, where the LORD my shepherd "makes me to lie down in green pastures" (Psalm 23:2; cf also Ezekiel 34:14-15). While the mention of green grass could just be a detail of fact, that is what it was and where they were, within a whole context of Jesus as shepherd feeding the people, how could there not be a connection?
So what then should we "understand about the loaves"? Mark shows us Jesus the shepherd who feeds his sheep (first through teaching, Mark 6:34, since "Man does not live by bread alone but by every word that comes from the mouth of God", Matthew 4:4/Deuteronomy 8:3), and that Jesus is the Christ, in one person the successor to Moses, the Davidic King, and the LORD God himself. This is what we need to understand by the miracle of the loaves.
Throughout the passage there seems to be a question of identity. Who is this Jesus, really? In the walking on water, again the closest Old Testament referent must be the crossing of the sea in Exodus, with the waters parted, a signature miracle of the LORD. So when Jesus says, "Take heart; it is I. Do not be afraid" (Mark 6:50), whether directly or indirectly, he is saying that he is the "I AM", he is the LORD their shepherd. (Cf also the connection between shepherding and walking through the waters in Zechariah 10:1-12 and Psalm 77:7-20.)
So that is why the grass is green!