Andy? Have we changed your mind at all yet?? :)
OK, so lots of points have been made on this and I see the logic on both sides. I'm not persuaded to change anything yet, but we'll see. First let me share my own thinking.
First, neither Black nor White appear in ROY G BIV so Rainbow is going to be a technically incorrect name either way.
But my real reason for pairing them as I did is for expandability. While a Rainbow Treehouse set expands Zendo nicely with either black or white pieces, a Xeno Treehouse set plays better with an IceTowers set if it has white. Moreover, when we eventually get around to releasing it, Volcano will probably have black caps, making Rainbow Treehouse as currently planned a better fit for that.
But maybe these reasons are all just so much noise. I think I could be persuaded to change on this.... I need to build out some prototypes and ask around.