My two cents: I think, it's mostly due to grey background color, text contrast and CRT rendering. In reverse order: * Not all colors lent themselves equally w

My two cents: I think, it's mostly due to grey background color, text contrast and CRT rendering. In reverse order: * Not all colors lent themselves equally well to a CRT, especially to the more cost effective ones. Green, esp. when in multiple shades, didn't render well (or, the other way round, issues with green are more easily detected by the human eye), so you won't see much green in early color UIs (or rather bright single-color expressions in elements like bars). * Also, CRT specific, you want to stick with primary colors (RGB), since even a small misalignment of the cathodes will result in blurry text in more evenly distributed hues. (Same is true for the outer regions of larger color CRTs with shadow masks.) * Monitor calibration wasn't always the best, shades of grey often exhibited a red hue, esp. on systems with a gamma of 2.0. * Considering what we have established, we are searching for a color which consists mostly of R, G, or B and renders well on an average color CRT on top of a grey background of #C0C0C0. (Also, mind that in 1993/4, we're probably not speaking of millions of colors to choose from, but rather more of a 4-bit color palette, if we're looking for robustness.) - Green, as already established, is somewhat complicated. Also, a mostly green color of comparable intensity is perceived somewhat brighter than, say a blue one. It will stand out against black text, probably more than you want, and it's contrast ratio to a light grey suffering from a red tint isn't great. - Red may not be the best choice either, as its contrast to grey isn't the best (ask your printer) – and its use may be best reserved for representing an active state. (There's also the cultural issue with red usually signifying limits or even off-limit areas, which isn't especially inviting.) - Which leaves blue for a passive state, which is actually a good choice. It's perceived slightly darker than the other primary colors, which is favorable for rendering text, it has a good contrast against light grey, and isn't affected by any missalignments or color calibration issues (as it doesn't share with red), and, while visible, isn't too distracting in what is mostly black text (you still want to provide for fluent reading of a given text, even, if it embeds a link). Moreover, it is more friendly to impaired vision than green or red (which are actually used in diagnosing defective vision). Properties, for which it had been used as a favorite color in UIs already. Moreover, on the cultural side of things, where red indicates restriction, blue indicates recommendation and instruction (compare traffic signs). So, I think, blue was quite a natural choice.