I'm on a Mac, using the light navy color scheme and FF -- I can't see the text at all even when I highlight it.
I suspect it has to do with how highlighting works on a Mac, though. When someone uses yellow or white text, highlighting it doesn't actually make it more legible when I'm on a Mac (even though it does when I'm on a PC). Highlighting doesn't change the color of the text on Macs, it just adds a light blue background.
Ferrets posted another way of getting truly illegible text, though. You can use the "glow" function (the G next to the strikeout button). So if you use the following code:
[glow=black,9,300]testing invisible text[/glow]
You get text that looks like this:
testing invisible text
Highlighting it ought to make it perfectly legible. You can also use other colors (the default color for glow is red), but you have to change the color of the text to match the glow color:
[glow=red,9,300][color=red]testing invisible text[/color][/glow]testing invisible text
The default setting for the "glow" tag is "red,2,300" -- you'll want to change the 2 to a 9, because some browsers end up seeing fuzzy colored text instead of a block of color. At "9" the text is fuzzy enough to be illegible until you highlight it (I think), but at "2" it's still legible.