Thanks for the feedback, everyone!
As Bruce speculated on the front page, Keith Goldman's work was indeed inspirational for me.
The stone is intended to be something like sandstone, which is why it matches the sand. The fact that I had all that tan sitting around waiting to be used contributed to the decision, of course.
While building this tower I realized I didn't have nearly enough grey arches, slopes, or bricks. I used just about all of my 1x4 arches for the windows. I have since spent way too much money on BrickLink getting lots more, as well as more tan, brown, and green for landscaping and blue for water. I just started another island to go along with this one, which should be completed in a week or two. The next island will have a lot of sandstone to match this one, but I'm throwing in some dark gray volcanic-rock as well to mix things up.
Look for that on my MocPage soon, as well as a set of detail and interior photos of the Island Tower.
Float like a weasel, sting like a weasel.