I've wondered the same. After asking around it seems that all iridescent chicken breeds call for a green sheen even though blue, purple and violet colors can be found. Sumatras have really nice green iridescence (and blue is seen as a fault and many claim that a Sumatra with blue iridescence is under stress), Ayam Cemani have nice blue iridescence and there are some pacific and South American breeds that are purported to have some Green Jungle Fowl in their ancestry that are "violaceous". Of course, Green Jungle Fowl are incredibly iridescent (and Bekisars, the hybrid of green and red Jungle Fowl are extremely iridescent purple/violet).
I had a breeding project where none of the birds had any iridescence in their bloodline (for quite a few generations anyway) and I had a lot of black birds show up with some iridescence. I crossed those birds to try and get something else and got a few chicks with even more iridescence (this photo is of the hen) so I'm crossing a pair together to see if I can get even more. I also have Sumatras crossed to Ayam Cemani and so far none of them seem to have more iridescence than the parents but perhaps after another moult. At any rate at some point they will all be crossed together.