Here's some code for in your script:
var %txt = <LI><b><i>noun</i>:</b> ...<blockquote><font color=green>Example: <i>"She checked several points needing further work"</i></font></blockquote>
var %regex = [color:blue]regex[/color]
var %result, %b = $regsub(%txt,%regex,\1[color:green]replacement text[/color],%result)
; the replaced string is now in %result
The %txt contains whatever you need to change, you can change replacement text to about anything, except \ or $ followed by a number or commas since those are special characters for pcre or mIRC. Yes, the \1 is needed or you'll lose everything before the </i> you want to change

Replace
regex with:
/(.*)<\/i>/i to replace last </i>
/(<\/i>.*?)<\/i>/i to replace second </i>
/((?:<\/i>.*?){2})<\/i>/i to replace third (=2-1) </i>
/(.*)<\/i>/i to replace first </i>
If you want to know more about regex, download pcre.txt (PCRE man pages)from
www.pcre.org since that's what mIRC uses (skip the first parts, interesting part starts at PCRE REGULAR EXPRESSION DETAILS). Google has some more general info about them, some mIRC scripting sites too...