Just use the color thingy to set the color of your text to... anything, then click the "Use BB Code Editor" button, then change the color code to EEEEEE (or eeeeee, or eEeEeE, or whatever). It's exactly the same color as the default background, so it's totally invisible unless you highlight it.
Like this! I feel like I should put something really creepy here or something... Muahahahahahaha!
Like this! I feel like I should put something really creepy here or something... Muahahahahahaha!