While it is difficult to be totally sure it is hair dye and not a lens filter, it appears as though Joaquin Phoenix's hair is green in that short video from Joker released earlier today.
Below, you can see a screenshot taken from about 2/3 of the way through the short video released online earlier today by Joker filmmaker Todd Phillips. In it, Phoenix's character Arthur Fleck seems to have colored his long hair green.
With a dark background and a generally greenish tint to the whole screen, it could be easy to miss in the half-second flashes of the Jokerized Phoenix, but a close look at the image shows that the green hair is as much a part of his clown makeup as the red smile.
laying with the exposure and brightness will yield more or less vibrant hair and makeup, but taking a screenshot from the video, without any modification (as seen above), and using the eyedropper tool in an image editor shows that the hair at the top right corner of the image is pretty clearly a shade of green:
Fans have wondered whether Joaquin Phoenix would actually end up wearing The Joker's iconic makeup in this origin movie, or whether we would go pretty much the whole film without seeing him come into his own as the Clown Prince of Crime. From this and other early photos, it appears as though an actual circus and real clowns will play a role in shaping his mad alter ego.
