Controlling Rotation Speed
The biggest effect you can have on the final result, aside from your selection of filament, is the speed of rotation.
In general, the colors change faster when the rotation is faster.
When the colors change faster you get narrow strips of color that spiral their way upward around the print.
The smooth gradient is caused because the color changed one full cycle in the entire print. This can be done more easily in a small vase mode print like this because it uses so little filament. A bigger print would need slower rotation to be able to have fewer full color cycles.
This is exactly the same 3D model (same gcode) but printed in a different filament with a different arrangement of pulleys. This one got two complete color cycles in the same print.
In this larger vase mode print the same combination of pulleys was used that yielded the single-cycle small bin, but this bigger vase used more filament, so it rotated at least five times. If one could make it spin 1/5th as fast, it would have gotten through one color cycle in the whole print.
You can load a spool in one of two orientations – either side can face the spool pulley flange. The spool rotator moves in the opposite direction depending on which side faces the spool pulley flange. So, you can make the spool rotator twist the filament in the opposite direction by removing the spool, turning it around, and remounting it.
This can make a big difference for filament that is pre-twisted (see below).
Working With Pre-Twisted Filament
Some makers of coextruded filaments pre-twist it to give an effect like the filament rotator achieves. It can still be used with the filament rotator and you can even take advantage of it for additional effects.
You can take advantage of pre-twisted filament by configuring your spool rotator to twist the filament in the same direction it is already rotating. You may have to try it both ways to see which way works better for your print. Reliable, very slow rotation is ideal for larger prints and prints with a lot of infill.
In one configuration the twist applied by the manufacturer will be reduced by the spool rotator, so the combination rotates more slowly than you could have achieved with the spool rotator alone. In the other configuration the two combine to rotate the color faster.