Items where the 2nd dye channel needs an adjustment.

[Caesius]

It feels to me that every gear piece has 15 dye channels. The first one is the primary color, but the rest are unsorted.
So sometimes the 2nd channel is the color of a zipper, sometimes a scarf, sometimes a button

This is literally true, almost. There's 16 color channels and any number of them can have the dye applied to them, and now there's a possibility for a second dye to apply to some of the color channels.

They did say that some items with two dye slots would still have much of the clothing undyeable due to the specific material used, however. The color channels aren't used for the entirety of the gear piece and there will often just be straight up colored textures for complex materials.