To make the magic white font, type the following, except replace use the chevrons instead of the [] keys:

[font color=white]text to be white-fonted[/font color]

Here's how it works:

I think that if Harry had said "Slytherin is a perfectly good House; I almost went there myself, and one of your godfathers is a Slytherin", it would have felt better to me. Instead, it was "they should be lucky to have you". (To which Portrait-Snape has, in this fanfic, the riposte we would expect.