Ooo it's coming along well! Hmmm for a dress colour, I think dark blue would look good, though I'm the type who would just paint it black with some kind of red highlights to tie in with the background =P
I sort of agree with the above comment...I was going to suggest something in the grey tones, to compliment the husky, because I feel as if colour in the dress would detract from the colourful designwork of the hair. However, I think having the entire thing done in grayscale with the touches of red is a bit pedestrian/commercial (red, black, and white has been overdone) so what if you did the dress in a gray-toned colour, like a slate grey (which has undertones of azure) or a dark or purple taupe (I see some taupe in the hair). I think it would make the painting more complex...You'd get your subdued palette without recourse to artsy-frank miller stylism.
I figured out what colour to use, but most importantly how much to use. Its gonna be grey-like and bright at the same time.