It's such a pleasure to be given the opportunity to critique your Joker fan art. Personally, he is one out of the top 10 villains on my list, and seeing you give justice to him based on *theharmine's craftsmanship is an epic win!
For starters, your foundation of colors is admirable, including the texture that blends with them. To take it to the next level, I'd say do studies on sharper shadows and in-depth detail of the hues - quite considerably on his striking lime green hair strands, you can apply a deeper green behind his ear and the back of his strands. You could also experiment more on the textured side of the artwork, and experiment with ragged/tainted feels to compliment both the Joker's insanity and the decaying atmosphere he's in.
In conclusion, I think you do excellent work with previous linearts. Don't be afraid to try something new with color/manipulation/proportions and always know that there are resources and tutorials on deviantART that help artists like you grow and find your own signature in art. Take care.