i havent perfected any of these methods, but they seem to be worth learning
for multi colors use your acetone mix and get the disc wet right before applying dye. the acetone mix will mix with the water and get a cool effect on the edges, while the disc is still wet apply your other color, the colors will mix alittle in the standing water. Rinse immediately. You will get cool effects between the two colors much like factory dyes.
if you like that method try using tape to create a reservoir on top of the disc (or just dye the bottom) and use more water for more mixing and more a tye dye effect.
and for shading i have tested (never to spectacular results but ive only used this method alittle) using a spray glue.
mask off your disc in your fav method (glue contact paper ect) lets say for and example a long rectange. you want this rectangle to be a value scale from black to white. the disc is white, and you want the left white. spray heavily on the left and slowly lessen the spray to the right until the far right has no spray and will be black. test with dry times for the glue youre using. I suggest not allowing the glue dry completly so it is allowed to run alittle. if done right you should have a pretty decent greydation.
for blending two colors, dye the whole area in the lighter of the two colors, and then spray and then apply the darker.
again i reccomend using the acetone mix because it will fill the pores between specks of glue more evenly. also a thinner dye tends to work better for the shading or greydation.
and i wish that I had a method of making a thicker dye that could be painted on, but i dont have one yet. im suck relying on my masking to keep from getting dye on areas i dont want it. Ive considered mixing flour or something similar to make a thinner dye, but im not financially in the best place to be spending money on alot of dyes and what not lately, but i will let you know if i get around to trying it.
I have heard of someone using "worm dye" its dye for dying fish baits made of plasic (lures and gummy worms ect) that is thicker than rit and water or acetone. i havent tried it personally yet.