To explain a little further for those curious, normal weapons, like a long sword, have the usual top / middle / bottom and corresponding colours customisation we're familiar with:
Whereas darts and most other thrown weapons are treated differently, and have hardcoded appearances based on their icons:
With how the engine scripting works, you'd either be able to customise darts/shuriken or the existing throwing axes we can. It would be either/or. The game only lets you distinguish by class ("throwing weapons").
It's conceivable to make a special class for just darts but well ... maybe you can see how this is becoming a rabbit hole?