Samples, clockwise from left: Navajo plied crochet and knitting, two-ply knitting and a skein of two-ply yarn, all in Sea Anemone (23 micron Merino).

To take advantage of the colour changes, I often spin just one ply and then navajo ply it - this gives a self-striping yarn which shows each colour on its own. You can vary the lengths of navajo loops to blend areas of colour or separate the different shades.  Navajo plying also produces a more rounded yarn (because it's effectively a three-ply) and tends to equalise slight differences in thickness. Although it is tricky to learn at first, it's also a fantastic way to spin up samples or use up unwanted bobbin ends!

Spinning with Heavenly Wools Fibres
At the moment I have two very beautiful combed wool fibres in stock - 21 micron Merino tops and the luscious Haunui Handcraft Wools, a fine NZ Halfbred coloured wool in a selection of overdyed tones. This fibre is 24.5 micron, and has a very soft handle similar to merino. It is very easy to spin

I occasionally dye silk, merino/silk and mohair; to see and purchase these, please visit my Etsy store.

Four-ply weight self-striping Navajo plied merino, showing the clear separation of colours possible using this technique.

A closeup of the two-ply lace yarn used in my Holden Shawlette.

Plying the colour mixes on themselves gives a heather effect. This two-ply merino yarn was randomly spun and plied, although of course it's easy to pick the colours out of the multi mixes and spin and ply them in sequence to produce a more orderly pattern as follows:

Split the wool lengthways along the whole 100gr of tops to preserve the sequence of colour changes. You can ring the changes here - make four lengths of equal thickness, for instance, draw the fibre out a little and spin two of each into singles, which will then ply on themselves quite accurately matching the colours. Or you could reverse the second set of lengths, making the colours work in opposite directions. This will make an interesting stripe sequence which would look great with socks or scarves!

Effect yarns are fun to try and can make some stunning items. 
I spun up the thick and thin single yarn used in this project using 23 micron merino tops  (no longer available through Heavenly Wools) to try the technique - then I felted it slightly to equalise the twist and to make it a bit more robust. You can see follow the project through to completion in my past blog posts.

Converting an old overwashed jersey to a new fashion item with the help of navy dye and a thick and thin merino yarn!

Corriedale wool tops dyed in the 'Bromeliad' colourway and spun thick and thin, spiral 'beehive' plied with a very fine dark 'Earth' binder single.

Another effect yarn I've been exploring is this thick-and thin plied with a thin high twist yarn - a 'beehive' yarn. This is my first attempt, so it has its good and bad bits. Tricky to get at first, the plying action is crucial to success. This type of yarn was pioneered by Jacey Boggs. After looking at this lovely yarn for a while, I was inspired to knit it up into a warm cowl and earflap hat - you can see photos of both these items in my Ravelry Projects: Wing and a Prayer hat and Round the Corner Cowl.

Sample skeins of undyed Navajo plied Haunui Handcraft wool - 3-ply (fingering) weight.