I love yellow sandals, but I am not a fan of yellow toenails under any circumstances because... well, they're yellow. While yellow can be a cute color for clothes and shoes and accessories, it is just an inherently bad color for toenails, IMO. If you want to keep things neutral, I'd go with a French pedi so as not to compete with the sandals. Otherwise a bright contrasting shade would also work. I just cannot get over a visceral, negative reaction to yellow on toenails.