...unless your post is the very first one in its thread.

What with the alternating shades of colour for every other post when people view multiple posts on a single page (doesn't everybody read zIWT this way?), you can't be sure if your particular post will happen to be in an odd- or even-numbered spot in the sequence. And therefore you won't know if the post's overall (default) background will be white or grey -- so whichever one you pick for the font-and-background colour of your "sooper-seekrit" message, perhaps half the time this will show up as a differently-coloured rectangle splashed across a background of the other shade.

HTH!