Painting noses is one of the most challenging aspects of bringing a painting of someone to life. The nose is the anchor of the facial structure and getting it right can make it easier to do the eyes, mouth, and cheekbone structure. The key to painting a nose is to find the shadows and lines necessary to define the nose and using those lines to enhance the other features of the face. It can be difficult finding this balance at first, but when you finally discover the technique, you'll be able to paint noses that will give your portraits individual definition.
Rough sketch the subject of your portrait using a reference photo if you need to. Is the nose hawkish or bulbous? Are the nostrils flared or tiny? Don't worry about details at this point. You can add those as you paint. The idea is to get a basic structure of the nose onto canvas.
start with the nostrils. Use a flesh tone to create the arc of the nostrils. Keep the paint dark at first, working from the nostrils to the bridge of the nose. Use some of your paint thinner to lighten the edges of the nostrils to help give depth.
Add a little white paint to the tip of your brush and work it along the bridge to create a little pale definition, then rinse your brush and add a light bit of dark brown to the tip. You'll use this to create depth around the corners of the nose, where it meets the face. This will create shadow and depth.
Use a light brown and white mix to lengthen the bridge of the nose toward the center of the eyes, making sure to use back and forth brush strokes when you reach the center of the eyes to create the slight depression often found at this point.
Take a look at your painting and decide where to touch it up. Add deeper flesh tones to enhance shadow around the nostrils and to bring the bridge of the nose up. Continue to apply these shades until the nose takes on its shape. From there, work away from the nose to create the rest of your portrait.