I have always found painting noses to be so fun! They are incredibly different from person to person. No two noses are the same. Getting the structure of the nose is so important. Understanding that there are planes is invaluable. The goal of the artist is to create a nose on the face that feels three-dimensional. The nose should feel as though it comes forward from the face.
The nose is basically a combination of a box and a ball. The bridge of the nose is more box like in structure and the tip of the nose is more ball like. The forms on the bridge a cleaner and distinct like the planes of a box, the ball or tip or the nose is subtler in form like that of a ball.
I try and look at the nose as a series of planes. Identifying the side planes, the top plane, the tip of the nose and the plane that drops under the tip of the nose as it descends to the mouth area of the face. Usually the side planes of the nose on either side can be a distinctly different value that the cheek plane. Sometimes this can be very subtle, especially in the light and may only change in temperature not value.I think nostrils are often painted too dark and lacking in color or temperature in many portraits. Look for the warmth in the nostril, especially on the lit side. Sometimes the nostril on the side with the cast shadow can be painted as one, showing no distinct separation between the nostril and the cast shadow, which is usually this cast shadow, is very warm. Look to Sargent for examples.
The nose is generally quite warm in temperature as a whole. This is obviously truer in lighter complexions, but can often be seen even in darker skin as well. The tip of the nose can be very warm in some people.
Follow the highlight as it comes down the bridge of the nose. It sits just on the edge of the top plane and the side plane of the nose closest to the light. It runs down the nose, following the edge of the plane, then breaks just before it gets to the ball of the nose. Then just below the long highlight, there is a highlight on the ball of the nose. These highlights are most often a lighter version of the main color of the nose. But not pure white!
Study noses. Make sketches both in oil and pencil. Try a variety of lighting conditions of the nose to gain a better understanding of its structure and unique qualities. They are fun, unique and a great lesson in structure, volume, temperature and edges.