I like plaster, probably because I grew up with midcentury houses and that's how a pool is "supposed" to look. :) I suppose a white pebble would look close enough if you find it's a better material. (I don't know if it is or not.)

Those pools with colored insides always look wrong to me!

If you haven't actually been in a pebble pool, you should probably try to find one to get in. Regular pebble is well, pebbley. Mini pebble is not supposed to be as bad. My girls and I play hard in the pool and we struggle with plaster tearing up our feet. When we have been in pebble pools we have had bloody feet. Maybe not an issue for you, but it was for me and I went back with plaster, knowing that I will have to do it again. If you don't mind spending a lot of money the bead tech seems to be the most smooth pebble option out there IMHO. I really liked it, but it was spendy... oh so spendy.