Here are two edits I did. By no means perfect, but rather a result of me messing around.
I think there is great potential in these portraits.
What they're lacking most of all is visual interest.

There is no sense of drama. Very vanilla. A memorable portrait exudes something more, a mood, an atmosphere.
This can be achieved through the colors, facial expressions, ambiance.
Whatever mood you want to achieve can be amplified by the colors. Just working on a white background and picking a very straightforward palette doesn't set much of a mood. It's hard to get a read on what emotions these girls are feeling, because the faces don't tell us much, and neither does the colors. Contrast also make the piece pop!

As talked about before your colors make very illogical jumps. Incorporating saturated and un-natural colors can make for an interesting effect, but if they aren't tied together at all with the palette of the rest of the picture we are left with something that looks pasted on and foreign. If you are shifting hues you need intermediate shades so that effect doesn't arise. To Make the piece seem more whole.

The hair is already an improvement! One of the weakest aspects in all of the portraits was the noisy hair.
However you must be careful not to let the hair become to ''clumpy'' or it will start to read as dreadlocks, my edit of the red haired girl also suffers a bit from this. A way to avoid this would be to work the hair in mostly flat colors ( see the turquoise girl).

You could also afford to be a bit more painterly! Think less in lineart, use blocks of colors to define and separate different areas.

The basic anatomy seems pretty solid across the board. There's just need for more flair and pixel tech!
Catch our eye! Make us feel!