Heh, I was just going to suggest ^k99 but I see codemastr just did. Transparent color? I know nothing about that... transparent would indicate something slightly more transparent than the value of $colour(Normal). I just changed my Normal Text color to Orange and sure enough, ^k99 looks orange. Another thing I discovered the other day, if you use 99 for a background color, it also displays the default background color. (This was slightly disappointing as I was trying to hide text using ^k99,99)
Other alternatives would be ^o (if you have no reserve against using ^o, and have no other formatting to lose). I'd personally use that before doing ^b^b or ^u^u. I only ever use that method if I want to swear on a channel or on these forums.
PS. I decided against speed testing this for today, got caught up doing outside work. sh[b][/b]it