I think the biggest challenge for me was the skin tone. My previous paintings are fairly high contrast and the colors are not that vibrant. Due to my drawing process of creating the grayscale painting first (this way I only have to focus on the values), when I add colors, I end up for the most part "tinting" the black and white underneath (eg. that rather flat effect you get when you add the hue/saturation adjustment in Photoshop and select colorize). This time, however, I spent more time afterwards actually painting with color, thus shadows and highlights are not as desaturated anymore. Lastly, this painting is not as sharp as I wanted it to be, because it turned out at the end that my document resolution was pretty low, so I had to resize and overpaint some of the details :s.