1. Try to use hard round brush on low opacity for shading. Look up manual blending (use color 1, paint over it with color 2, colorpick result between them, use that color, rinse and repeat, see A). 2. Try to use airbrush to lay rough shadows and then refine them with hard round brush + blending (see B). The shadows become sharp the closer the objects are to each other and the more intense the light is. E.g. nose drops sharper shadow over the lip than chin over the chest. 3. I think what this painting actually lacks is more color variety (don't use darker color for shadow, try to change the hue as well, e.g. violett/dark red instead of dark salmon; looking up color theory will help you a lot here). Another thing is refinement - it is something that comes with experience and practice (e.g. how to render eyes - eyelashes directed outwards in curves, adding tear ducts etc.). Try to look up real life references. It is a cool portrait, just needs few tweaks. Good luck! | Artwod Feedback