Decoder cards, help needed

If memory serves, it's just pink pattern on top of pale blue text, normally. You can't get good patterns in Paint, though. There are some good free paint programs out there, and you can get Paintshop Pro very cheaply (I commonly see it on sale for like $39 with $39 in rebates, making it $0 after 6-8 weeks).

You'd type what you wanted in pale blue text, put a red/pink pattern over it so that you couldn't really see the type anymore, but just barely, use the program's Multiply setting for that top pattern layer so it doesn't obscure your text, and save it out as whatever format you want for Office (like BMP). "Multiply," by the way, means that wherever the pink/red and the blue touch, you'll get a mix of the two, but when the red is effectively subtracted out by the film, the text will look normal.

You'll then have to play with both the blue color and the pink/red color, as well as the multiplication, until you achieve the right levels of obscurity and clarity.

Does that make sense?