The idea is that your heel and toes can still grip the mat, but your feet don't get as cold as they would barefoot.
The other part of the challenge is to use up as many bits and pieces of yarns in your stash as possible. Socks with a lot of very different (sometimes clashing) colors are sometimes called Monster Socks on Ravelry. I have a big stash of sport weight yarns in many colors:
Most of these balls are handspun yarn, although there are a few mill spun balls in there.
One problem with making socks from handspun is that my handspun wool does not have any nylon mixed into it. Nylon makes the socks wear longer without getting holes. In the past, I have tried to card nylon with wool to make a sock blend, but it didn't blend in very well and I didn't like the result. So my new idea, to make the socks more durable, is to full them before wearing.
Fulling is just washing the socks in hot water and perhaps tossing them in the dryer, to make the wool fibers catch on and cling to each other more tightly, otherwise known as shrinking. It is like felting, in that the objective is to make the wool fibers fuse together, but felting is done with loose unspun fibers, whereas fulling is done to woven or knitted fabric. It was common to full woven fabric in pre-industrial times; in Scotland, there were "waulking" parties, where wet wool was beaten over and over by gangs of women around a table, as they sang songs called "waulking songs." Nowadays, all we have to do is throw a piece of woven or knitted wool in the washer with some hot water and some other pieces of clothing, agitate in the machine, and then remove and check the degree of fulling. Further fulling can be done in the dryer, as the semi-wet piece tosses around in the humid heat.
Fulling makes knitting much more durable, so it seems a natural for socks.
I plan to knit fair isle yoga socks with corrugated rib at the top and along the edge of the opening at the heel and toe. After fulling, I'll have an idea of the gauge of my fulled knitting--the number of stitches per inch after fulling, as well as before fulling--which will be useful information in designing future fulled socks or other clothing. Ideally, I could come up with a percentage rule--i.e. "make your sock 10% wider and 15% longer than you intend for it to be after fulling"--that would work for most wools.