Let’s say you keep track of passwords and other login information in a text file. (You store the file on an encrypted disk image, of course, to keep it truly safe.) And let’s say you want to have the document open on your screen without its contents being visible to casual passersby. Here’s a solution:
Create your text file in Rich Text Format in an application like TextEdit. Enter some login details on a single line, such as: Server username/password: Bob/$w0rd. Next, place periods and some amount of white space on either side of the username and password, so it looks like this: Server username/password: . Bob ./. $w0rd. Next, highlight the username (Bob) and select Show Colors from the Format > Font submenu. Use the resulting colour palette to change the text colour to white. (Don’t change the colour of the periods.) Do the same for the password ($w0rd).
The line will appear: Server username/password: . ./. .. To the casual viewer, the space between the periods appears blank. But if you need to see the info, highlight the space between the periods; your text will appear as white within the highlight colour.