Begin by looking at the wine in good light against a plain, white background. Hold the wine to the light and view the color and clarity of the wine as the light shines through it. It should not be cloudy or hazy. Next, look down on the wine and notice the hue of the wine, especially around the edges. Also notice how much color graduates from the center of the glass to the rim.
White wines vary from almost colorless, to hints of green or yellow.
A brownish tinge is a sign of too much oxidation, caused by contact with the air. Reds tend to tell more in terms of age and quality by their color. Reds become paler with age. The rim of the glass is where to look to get a feel for the age of a red wine. The paler and more brown, the more mature.