One of the benefits of working with an indexed palette is that you can have the same colour more than once in the palette, this essentially means you can write stuff in solid colour areas and then change the colour of the text to the same colour as the solid to hide the text.

There are some really good examples of this in the demoscene history, where it was pretty common practice. There's a gallery of hidden messages over at GFXZone.

http://www.gfxzone.org/theme/hidden/hidden-personal_01.html