These two characters are from one of my favorite cartoons. The Batman (2005-2008) was known widely for its take on enhancing the visual features of the supervillains; this style was cultivated by character designer and later producer Jeff Matsuda.
Harley Quinn (front and center) and The Joker (rear left) are seen together with this pose of Harley Quinn taking inspiration from her official character model. The reference for The Joker was taken from a screenshot of a scene in the first episode.
A series of coloring pencils and thin-pointed coloring markers were used on a sheet of 22.8" x 30.4 paper.
First, a light sketch of both Harley Quinn and The Joker was made.
Second, colors based on the official character color pallets were added.
Some aspects of the line art were enhanced with thin-pointed coloring markers while the buildings in the background were lightly sketched in.