99 is the default colour, not necessarily equal to $color(normal). It depends on what you specified as the colour parameter on echo/aline/etc, which is $color(normal) if not specified. It does almost what a closing ^k (or ^o, ignoring other ctrl codes) does, except that a closing ^k also blocks background colours, while using 99 you'd need ^k99,99 to do the same thing.
This is why Khaled calls it transparent, I guess.
