Here's a piece I started from a sketch in my sketchbook - a card girl from photo reference and a boxer I drew from my head. The rest of the charcters were also from my head, but the background was referenced from theatre lighting - a bit different from what normal boxing event lighting would be. I was looking for more atmosphere.
The overall palette was derived from a mixture of sources.
here's the original sketch:
...and finish in Photoshop CS: