Cheers to Dustin Chabert, Jeremy Eubanks, and Rachael Ryan for last week's responses on Extended Maceration!
This week: Red Wine Color in Tasting
From a blind tasting perspective, discuss the various elements that can impact color in red wines.
I'm not quite sure if I'm answering this correctly, but I'm going to take a swing at it. First (and probably most obviously), the varietal chemistry and makeup will show a lot on sight alone: thin-skinned grapes with a lower anthocyanin content will yield lighter-hued and clearer wines, where thicker-skinned grapes with higher anthocyanin content will be deeper in color and more opaque. Certain varietals are also known to show specific secondary colors, especially in the rim (magenta with Malbec, orange-ish with Nebbiolo).
After thinking about varietal, age factors in. Youthful wines tend to be brighter and have a more consistent concentration of color. The more bottle age the wine has, the more the color will tend towards garnet/tawny, and the rim variation will be more pronounced. The appearance of haziness or sediment in a youthful wine can be due to the wine being unfiltered, whereas a more developed wine will show sediment due to the tannins falling out of suspension.
Then, there are winemaking factors to consider. Piggybacking on last week's topic, cold-soaking can increase extraction of color, whereas extended post-fermentation maceration will lower the final anthocyanin content because they get reabsorbed by the skins. Likewise with whole bunch or whole cluster fermentations--they tend to lower final anthocyanin content. Co-fermentation with white grapes can also deepen the final color in red wines.
Then there's the tearing, which I've always thought to be indicative of wines of higher alcohol, but is actually due to the Marangoni effect, which (to my understanding) has to do with the surface tension between the liquid and glass vis a vis the rate of evaporation of ethanol along the meniscus... Anyway, the common understanding is that tears that are thicker and slower to form are related to higher alcohol and/or levels of residual sugar.
Put all these things together, figure out who uses which winemaking technique paired with which varietal(s) in which region of the world aged for which period of time, and voila. You've deduced the wine based on sight alone.
Fernando Beteta will often blind students on sight alone (sometimes, even putting plastic wrap on top of the glass so as to eliminate the temptation to sniff), making us go through the deductive tasting grid in its entirety before then being able to smell/taste the wine and make the appropriate adjustments.
To elaborate a little on the effect of age on the color of red wine: essentillay, as a red wine ages, the tannins will gradually react with anthrocyanic pigments in the wine, and as time goes by, they will create what are called polymeric pigments, which visually is lighter and more brick-like in appearance.
A few other points to tack on to Dustin's excellent response:
Color fixation in a red wine is mediated by a couple of different factors: first off, the quantity in the skins of the grape (i.e. malbec, tannat, p. verdot, etc.), but in order for that color to make its way into the finished wine it has to be 'fixed' or stabilized. Tannin binds anthocyanins, so the amount of tannin in a red wine can also impact its color - the chemical mechanism of viognier/syrah cofermentation color stabilization in the northern rhone (i.e. viognier is a highly phenolic, read: tannic, white grape - it provides the extra phenolic oomph to bind the anthocyanins of syrah). However, that reaction is also significantly impacted by must acidity, with the anthocyanin binding reaction hindered by high-pH (i.e. non-acid) musts.
TLDR: high-acid wines, even from highly tannic grapes (like Sangiovese and Nebbiolo) tend more towards the red side of the spectrum than the blue, because the low must pH prevents as much of the color from the grape skins stabilizing into the finished wine, and that color instead precipitates out as sediment at the bottom of the tank or barrel. How does this tie back to blind tasting? As much as everything is on a spectrum, coupled with an accurate assessment of skin thickness and maceration from the staining of the wine, one can begin to form ideas about acid as well. These three elements (skin thickness, acidity, alcohol) collectively are a wealth of information towards the identity of the wine, even before the first whiff is had (re: Master Beteta's exercises). If anyone wants more detail on this whole rat king of chemistry, Clark Smith's chapter "The Vicinal Diphenol Cascade" in his book Postmodern Winemaking is a great (if not necessarily thrilling) read.
The only other area to expand on I think is assessing not only the scope of rim variation (i.e. purple core to pink rim, or red core to orange rim), but also the intensity or width of that rim variation - is it a gradual even fade or a pronounced, sharp drop off? I have found that wines that see extended barrel aging, usually on the order of 24 months plus in small barrels, tend to have a very narrow band of pronounced rim variation - which I most classically see in wines like Rioja, Brunello di Montalcino, and Bordeaux.