Given that numerous early Web clients were text-based (especially www and lynx), there's the question of what colours were available on a (possibly) colour-capable display, or a colour-aware terminal emulator.
These would have been provided through ANSI escape sequences, which allowed for white, black, red, green, blue, yellow, magenta, and cyan.
With a bright background (white), blue is the most clearly-visible non-black colour, and avoids R-G colourblindness issues. On a dark background (black), cyan is light enough to be readable, but not as glaringly distracting as either red or yellow.
Given the prevalence of green and amber phosphor displays, and some adoption of that colour scheme for terminal colours, yellow and green might have been avoided due to conflicts with standard text.
The next question is when colour support was added to terminals. I'm confident that xterm could have foreground/background colours specified by 1997. Support for 16 colours didn't hit xterm until 1996:
These would have been provided through ANSI escape sequences, which allowed for white, black, red, green, blue, yellow, magenta, and cyan.
https://en.wikipedia.org/wiki/ANSI_escape_code
With a bright background (white), blue is the most clearly-visible non-black colour, and avoids R-G colourblindness issues. On a dark background (black), cyan is light enough to be readable, but not as glaringly distracting as either red or yellow.
Given the prevalence of green and amber phosphor displays, and some adoption of that colour scheme for terminal colours, yellow and green might have been avoided due to conflicts with standard text.
The next question is when colour support was added to terminals. I'm confident that xterm could have foreground/background colours specified by 1997. Support for 16 colours didn't hit xterm until 1996:
https://invisible-island.net/xterm/xterm.log.html#xterm_39
rxvt may have had colour support earlier. I'm unsure of dates or changelogs though it's mentioned in print as of 1994: https://www.google.com/books/edition/The_X_Resource/RKhFAQAA...