1 wait at least 24 hours before applying the next color
2 apply a coat with out over brushing the fresh wet dope. Put the dope on, leave it alone. The more you mess with it the more mixing occurs. If this means you put a coat on thin, then require a second coat after 24 hours then so be it.
3 you can use a clear dope layer as a tie coat if you continue to have problems. Put the clear on where you intend to apply the next color, wait 24 hours, then apply the color over the clear. This will isolate the layers some and prevent mixing at the cost of weight.
That's all I have on this. Hope it helps.
Phil