Holts sells their own label of coloured socks but they are very short and the material has always broken down on me within three months developing holes in the heels. Some of the Paul Smith swatches are great to look at but I've yet to try a pair on. I swear by Pantherella (Mr. Kabbaz sells some on his site) sea island cottons. As of now I've only owned the solid, ribbed variety but the colours are beautiful to my eyes.
I just received three linen pocket squares (about 14"x14" I think) by Ben Silver. I wear nearly exclusively linen as I find they fold better to my taste. I do have a couple of Dion silk pocket squares I've purchased at both Holts and Harry's that have beautiful colour to them. Canali makes nice linen squares too. I also own the Paul Smith "naked lady" white cotton square that's held up nice. I always try and manage a straight fold and have a little tuft of her (head) hair popping out my pocket as I'm pretentious like that.