I agree with BD, save that I don't think it's the addition of the tonal changes/shadows/hightlights, but the placement of them. There's no real definition of her clavicle or her neck which would go a long way in selling the realistic quality of the lineart (moreso that extra definition of her cleavage).
That being said, the colors are fantastically chosen. Even without seeing the tat, I instantly knew who I was looking at.
Very nice. I think you have a good handle on keeping your colors clean and smooth. This piece here I think may have called for some variation in skin tone, the addition of shadows, and some texture. I was real happy to see that you added some highlights