The gentleman on the right of the above photo appears to be wearing a grey/green v-neck, which looks great under a double-breasted suit jacket. I find that reducing the amount of visible shirt sometimes improves an outfit, especially with a two-button suit, and a v-neck is a nice substitute for a waistcoat. It also has the advantage that it allows you to add a softer texture to your outfit. In fact, I quite like the idea of very bright jumpers underneath suits, but sadly the more interesting colours seem only to be available in the sleeved versions, which are less practical under a jacket.
Most of mine come from T.M.Lewin because I like the fact that they edge of the v-neck is not too thick but, as I said, they're only available in blue, grey, brown and black. If anyone finds a good source of brightly coloured sleeveless v-necks, do let me know!