The Joker’s trousers were obviously a purple/green (actually kind of a teal/turquoise) plaid with some black and a tiny bit of orange thrown into the mix.
Although the subtle details of the fabric weave were seldom discernible in the actual film, here are some excellent photos of the screen-used Joker trousers on display – again courtesy of Paul Wares.
And here are a couple excellent close-up photos of the trousers, from an auction listing:
Here are a couple additional screen-used Joker costume display photos (which I’ve cropped) in which the waistband is directly observable:
(Note that the waistband was cut on the crossgrain, and care was taken to properly align the plaid patterns at the center front.)