Maybe it's the time spent poring over mountain bike magazines during the ano-crazy 1990s, but I'm really taken by the oil slick (or, to date myself, ti-dye) anodizing that's making its way from the BMX world into the less-trendy mountain, gravel, and road scenes. Named for the range of hues you might see in an oily puddle, the gold-green-blue-purple fades are most commonly seen on titanium.
We made these. We'll send you some if you want.
More ambitious manufacturers can achieve a similar effect by coating an aluminum or steel part with titanium nitride via physical vapor deposition or chemical vapor deposition (much like those gold drill bits you see at the hardware store) and anodizing that coating. Not only does the process look good, but it also increases the base part's surface hardness and, if all goes well, long-term durability.
Given their hundred-year warranty, you probably won't see any oil slick anodized J.Guillem frames. After all, a big part of the bikes' appeal and durability is their timelessness. That said, we're not afraid to dress our builds up a bit and think that the exuberant oil slick finish looks amazing against understated raw titanium tubing.
But maybe that's just the '90s talking.