If the rendered brown color happens, I'll get one. I like brown as a color and the colors Apple uses always look better in person than in renders.
Here is a chart of possible colors:
No real brown, closest looks like maybe 40mm. At some point the durability comes into play as well
While not difficult to make it gold, it is difficult to make it gold but not tacky. They could easily make it a gold mirror.
The challenge is anodizing titanium only can produce a small range of colors as opposed to say, aluminum becasue the process are different. Titanium's colors are due to the depth of the oxide layer produced by the anodization voltage, where as others use dyes during the process. That's likely why there is no red 15 Pro.