Very nice use of color!
I love how you meshed the abundance of grays and purples, and added in some crisp, bright orange/mustard yellow and deep red; overall all of the colors compliment each other.
I love how you drew the two people. Their expressions are great and their clothing creative and relevant to the "Halloween vibe."
I like the shading you did on their clothing, but felt that their faces could use a little more in some spots, however, that isn't all that important or necessary, just my own little nit-picking!
Overall you did a great job and really captured the mood of this Halloween piece. You truly have great illustration skills, and I feel you'd be pretty awesome at comic books or something. Anyway, keep up the good work!