First, thank you all so much for all of your supportive feedback. Wow.
And in defense of my sock pal - she did not say she was picky about yarn at all. In fact, she is open to anything. But her blog is extremely impressive and shows she is a true connoisseur. So I am all the more anxious to come up with something classy.
With this in mind, I am asking for more advice.
After all of your support, I have been swatching away with my chosen colors. I am drunk on these colors. I can't believe I had this yarn dormant in my stash. Against the black, the blues just sing out. Unfortunately I am not having much luck photographing it with my old camera. But maybe this gives some, blurred sense of how pretty it is. Here's Swatch #1. (Note: I made up a pattern as I went along, just to see what a pattern would look like - I would never knit anything this wonky into a real sock!)
I loved this swatch after I made it. But it is also a dense and unyielding fabric, and not very sock-like. Then, of course, I realized that with mosaic knitting, I was going to get a whole different texture with a flat swatch than I would with a swatch in the round. I also realized that horizontal stripes were going to be a lot more malleable than vertical stripes. So I made this (again, with a make-it-up-as-you-go pattern inlay just to see how it would work):
I love this even more. I do note that the mosaic section pulls on the yarn a bit - I will have to design around that. I think I am going to do some kind of medallion over horizontal stripes on the top of the foot, and then on the leg do an all-over mosaic design.
So here's my new question. I love, love, love these colors and will definitely make socks out of them. But it still makes a very dense and heavy sock - this is clearly a winter sock. I wish I could post a sample for everyone to feel, but failing that, here's my question - should I make these for myself and make something light and airy for my sock pal? Would you be disappointed to get a heavy sock in the sock swap, rather than something lacy you could wear with sandals? (I am actually not a big fan of socks with sandals, but you know what I mean. And I lived for 11 years in Germany, where the style is not uncommon.)
Advice welcome. And thank you all again for your comments. I am new to this blog thing and not sure about the etiquette of replying to posts that say "no reply" in the address field, but please know that every single one was very much appreciated.