If I understand correctly your color sliders shift from grayscale 1-color to another color mode like CMYK or RGB with more sliders.
Not sure what steps you take after the envelope, but sounds like you keep going back to the default graphic style. You can redefine that to grayscale, and your sliders will stay 1- color.
Draw a rectangle, change the color to grayscale, then option drag on top of the default graphic style to redfine.
I start with seven different colors "along the stroke". Several paths in an envelope that I then warp. I'm either ending up with 14 colors because of the warp or because the Eye Dropper is adding the colors (as I experiment) instead of replacing them. I'm not sure where I am picking up all the extra colors. It makes the artwork look flat probably because Illustrator is trying to balance something when I'm wanting to create diversity.
Additional color nodes (Gradient Sliders) would appear along the horizontal in the Gradient pallet. They would be added automatically when I either sampled the warped strokes or after I warped the envelope. But right now I have moved onward with a work around. If the problem shows up again. I can start a new thread. I can try to help you understand more if you want. Does that help?