Very beautiful work, Catherine. You have to be true to the expression of the model. It appears that you were, without having seen a reference photo. Perhaps you could post one.
If you want to lighten the expression, it should be done on the model's end. Perhaps you just have serious models. If you are painting from a photo, the photographer (perhaps yourself) is the influence that brings out expression.
Nothing wrong with serious. But if it bothers you, try using Earnie or Big Bird behind the camera on your models!
