This image depicts the most commonly-found stylesheet colors on the web's top sites—Paul Hebert did an amazing amount of analysis and this is just one of the intriguing visualizations he came up with.
Most of these are obvious staples, especially HTML red and blue, though it's interesting how far the blue "cluster" is from the default blue hue, whereas the red cluster merely modifies the saturation and lightness. This might be influenced by various "studies" of the most effective link color.
The odd thing is the popularity of #d2b48c (triggered by the "Tan" HTML color name), which appears to be the single most popular nonblack color after #0000FF (HTML Blue) and #FF0000 (HTML Red). Google uses it somewhere (though I don't see it) Is everyone just following the leader? (UPDATE: see below)
UPDATE: Hebert explains the Tan thing in the comments.
UPDATE: 10/22/2016. Hebert's >updated his method to exclude false positives (including the mysterious Tan)