Digital, painting portraits is one of the most difficult subjects you can paint. It has been said that if you can paint portraits, you can paint anything. The reason for that statement is because an artist is challenged for his drawing skills, his sense of colors and every one of its subtle changes with tones and hues, lighting, not to mention his skills with brush styles and techniques. Having said all that, my favorite portrait of yours is 
It shows me that you aren't afraid to leave your signature brushwork and you aren't afraid to use color. If you are trying to paint portraits in a stylistic fashion I would suggest you push that style even further than your samples show me. Stay with this style. If you are trying for a realistic portrait, you have a way to go.
Watch your angles on the face. Eyeballs are half hidden by eyelids and always the top of the eye is in a subtle shadow created by the upper eyelid. Its all of these fine points you have to deal with when painting portraits. A person's likeness begins with the eyes and mouth. But I like to tell my students that if you paint a person's form using light and shadows without any details, you could tell who it is. So using that as your scale, always paint the form first, then go for the details not vice-versa.
