Has anyone experimented with adding a reroll option that randomizes color slots for assigned parts? Or is that something too complicated to mess around with?

Dresser has no option for this, but it would be nice.

In the meantime you can use Mastercontrollers CAS and change the colors by random there for each part seperatly
MasterController-FAQ-CAS#What-does-Mast ... nge-in-CAS

Read point 7, 8 and 9.

You also need the integration module of MC.

Maybe I am missing the usefulness of this. If an asset has multiple channels, they'd all be different colors and that'd make for interesting fashion choices. Unless you'd rather it set all channels to the same random color?

I understood it like when I click a sim and use dresser to reroll top colour (or texture too would b nice) or bottom or dress
- to reroll it random like for Accessories or Makeup.

Without to use MC Stylist or CAS. This would be a nice addon I think