On the picture Moiraine's a bit light for black hair and Nynaeve is pretty light for a brunette. I even wondered for a minute if Nynaeve couldn't be Elayne, until I remembered Nynaeve has shoulder length hair now.

That's a scan of the painting (or a digital file - I haven't checked if Whelan works digitally or not). It's not very reliable, color-wise. Printing will darken and densify the colours. Irene Gallo may or not have applied filters to the colours on the RGB version to make them closer to how the CMYK image will appear in print (but on monitors not calibrated for that, it would have made it rather dark for web use - that may rather be the colours as they will appear on sites like Amazon etc). If the image is calibrated for the web, the painting once printed on the cover will be more contrasted and darker, giving Moiraine her black hair and Nynaeve brown hair.