How about making an exercise from painting female characters with watercolors? Yaaay!
To be honest watercolors aren't the easiest technique of making portraits. Drawing realistic people requires a very high level of accuracy, while watercolor technique is quite often perceived as series of "controlled accidents". When I googled "watercolor portraits", the vast major of searching results presented a very painterly style - sort of unfinished faces with colorful splashes around them. But the good news is that there were also a few extremely accurate artworks!
This exercise was made in a way I usually paint buildings. The only difference was that this time I decided to follow strictly photo references (most of my buildings are painted totally from imagination). Special thanks to stock models: http://faestock.deviantart.com/ http://nastiaosipovastock.deviantart.com/ http://mirish.deviantart.com/ and http://chirinstock.deviantart.com/ !
The first step is a pencil sketch. The more accurate it is, the easier it will be to paint over it with watercolors. Tool: a mechanical pencil (0.5 mm, lead hardness H) - it should prevent from smearing. Luckily I chose yellowish paper, so the coffee stain is almost invisible :)
To achieve a skin color I mixed yellow and red in different proportions along with adding different amount of water to it. For shadows I was also adding a little bit of violet. This brush appeared to be very useful. I used it for the whole work (except the background and some clothes):
Just as in case of drawing buildings, I used a thin waterproof pen (size XS) for making contours. Metal details were highlighted with a white opaque marker. I was using it for the first time so I had no idea what kind of results should I expect. For me it's like a very thin corrector (0.7 mm):
During such exercises you probably won't have an opportunity to fully express your creativity or move into the world of imagination, but hopefully it will improve your attention to details, proportions and light. Strongly recommended from time to time!