Every part was scrubbed with soap and water using a toothbrush or abrasive pad.
They were then moved into a lightly boiling pot of black Rit Dye.
Immediately after soaking in the dye, the parts went into this cold water bath to seal in the color and wash away excess pigment.

Then, they were moved onto a paper towel to air dry overnight.

I assembled one of the rear wheels, just for sake of comparison. The fresh wheel on the left really did look just like the dirty mess on the right just a few hours before.

Not bad, right?! I can't believe those old RPM wheels took the dye so well.