Focus in turn on hue, intensity and clarity. Each requires a different way of looking. The true color, or hue, of the wine is best judged by tilting the glass and looking at the wine through the rim, to see the variation from the deepest part of the liquid to its edges. Intensity can best be gauged looking straight down through the wine from above. Clarity—whether the wine is brilliant, or cloudy with particles—is most evident when light is shining sideways through the glass.

Each of these elements reveals different aspects of a wine's character and quality. But don't forget simply to enjoy the wine's color. No other liquid is as vivid and variegated, or reflects light with such joy and finesse. There's good reason wine's appearance is often compared to ruby and garnet, topaz and gold.

Next comes the swirling. This too can feel unnatural, even dangerous if your glass is too full and your clothing brand-new. But besides stirring up the full range of colors, it prepares the wine for the next step, the olfactory examination. The easiest way to swirl is to rest the base of the glass on a table, hold the stem between thumb and forefinger, and gently rotate the wrist. Right-handers will find a counter-clockwise motion easiest, left-handers the reverse.

Move the glass until the wine is dancing, climbing nearly to the rim. Then stop. As the liquid settles back into the bottom of the glass, a transparent film will appear on the inside of the bowl, falling slowly and irregularly down the sides is the wine's "tears" or "legs." "Experts" derive meanings from them as various and profound as fortune-tellers do from looking at tea leaves, but in truth they're simply an indication of the amount of alcohol in the wine: the more alcohol, the more tears. Remember that when you're considering whether to open another bottle.

Text from the Wine Spectator Pocket Guide to Wine.

Looking at Wine: Advanced Color Clues from Red Wines
A wine's color gives many clues to its character. First, color reflects the specific variety of grape (or grapes) the wine is made from. Take two common red grapes, Cabernet Sauvignon and Pinot Noir. Cabernet berries are typically smaller, with thicker, darker skins, than Pinot Noir. As a result, wines based on Cabernet tend to show darker colors, leaning toward purple and black, instead of the ruby tones associated with Pinot.

Second, color is influenced by growing conditions in the vineyard. A warm summer and dry autumn produce grapes that are fully ripe, with a high ratio of skin to juice, resulting in dark colors. A cool summer or a rainy harvest can result in unripe or diluted grapes, which will show up in colors with lighter hues and less intensity.

Vinification techniques can also affect color. When red wines ferment, the grape skins are left to macerate in the juice, like a tea bag steeping in warm water. The elements that create color, the anthocyanins, are found in the skins, not the juice itself (most grapes, even red varieties, have clear juice), so the longer the skins steep, the darker the color will be. Even after fermentation is over and the skins are discarded, some solid material remains in suspension in the wine. Some winemakers choose to remove this material, through fining or filtering; others believe the wine benefits from a little residual deposit.

Time in bottle—the inevitable process of aging—also has an impact. Young red wines are full of anthocyanins, and so their colors are deeper. With maturity, these coloring elements evolve, lightening through red to colors described as "brick" or "amber," slowly combining and falling out of suspension in the wine, creating a sediment in the bottom of the bottle.

So if you pour a glass of red wine and look at it closely, you may find a deep garnet color, with good intensity but not brilliant clarity. You might reasonably infer that the wine is made from Cabernet Sauvignon grown in a warm climate, that the winemaker chose to extend maceration and to filter only lightly, and that it's from a recent good vintage. If the tasting's not blind and you already know what the wine is, you can compare its color with what you might expect: Perhaps it's exceptionally dark for a weak vintage, indicating good grape-growing or winemaking abilities, or maybe it's already faded for its age, suggesting that the grapes lacked concentration, or the winemaker was unable to extract the intensity that allows wines to mature with grace and complexity.

Text from www.winespectator.com. Image from Wine Spectator's Pocket Guide to Wine.