I was somewhat involved in the arranging. People were out of their cars talking to each other, and it wasn't always easy to just grab the right driver for the right car to move it where you want, so there was a bit of "hearding cats" involved in juggling the cars into some interesting spectrum order. There was also a discussion if to keep like colors together, and if not, how to best separate them. I think you could say that it was a quick attempt to realign the cars in an interesting order with multiple inputs on aesthetic choices and some compromises to just get things together in a hurry.
So, not perfectly planned, or orchestrated, but a valiant effort by all to make a pretty photo op.