A portrait is always a great subject for exploring color combinations because the face often has a complex array of color values within it. When exploring how to use the correct color combinations, you should always consider what kind of effect you want. If you want to create a warm sensation, you will want to have warmer colors, such as reds, yellows, greens, and oranges. If you would like to keep the drawing more cold or brooding, a good color combination would be colder colors such as blues and purples. However, you can explore giving different features of the face warmer or colder qualities by color choice. You can also blend colors to create more variety within the gradients on the face. It's a matter of exploration and intention, but thinking about cooler or warmer color choices is a great way to make decisions.