Most of the info on the InterWebz seems to cover techniques meant to keep the colour changes in the roving mostly intact. This includes things like "fractal spinning" and chain or navajo plying. I don't use these techniques much because this isn't the look I'm after. I don't want stripes.
To mix up the colours in your roving, I guess you could split it up into the coloured bits and run it through a drum carder...but a) this would destroy the beautiful combing job that the top has, and b) assumes you own said carding tool. I don't. So what to do?
[initial roving, space-dyed]
Enter "combination drafting".
step 1) split the top lengthwise into 8 long, thin strips. This is usually easier than it sounds, especially with commercially combed tops. Pindrafted stuff is harder to deal with, and won't split nicely, but you can still try this. Carded roving won't split this way at all, so you probably cannot use this technique.
step 2) tear these thin strips into shorter lengths, with different colours at the "head" of each striplet.
[roving separated into striplets]
step 3) pick 3 short strips and hold them together. Start spinning, drafting across the top of the 3 striplets to mix up the colours. You do this by drafting from one striplet for a while, and then you bring in another one by tilting or rotating you fiber hand to allow the single to touch the new striplet. This should "suck" it into the drafting zone and naturally lead to the drafting zone moving to the new striplet. You can help it along by pulling fiber from the new striplet, to feed into the drafing zone.
[hold 3 striplets and draft from each one in sequence]
In this way, you can control how much of each striplet to use before moving to the next one. In general the colour shifts will happen much, much faster than if you were to simply spin from the whole top. And because you can pick different colour sequences by choosing different striplets as you go, you break the colour sequence that the dyer established. You'll end up with a mixed-up single.
Then, when you ply, the more plies you use, the more mixed-up it gets. There's no "order" to the colours anymore, and you get a more "optically blended" look to the yarn.
Here's the finished yarn (3-ply) of the roving in the above photos. It's pretty blended; the real proof, however, is in the knitting.
[combination drafted 3-ply yarn]
Here's a photo of another skein that I did using this technique, knitted up. You'll see you still get striping, but the stripes are narrow and random.
[combination drafted yarn, knit up]
I think this technique works really nicely for "tonally" dyed rovings, where the colours are from the same family and aren't too contrasting. It will have a more muddying effect on rovings that are dyed with high contrast colours (ex. rainbow-dyed).