This is a good way of making sure your site will be readable to people with less than perfect eyesight. First, open your site and press Print Screen on your keyboard. This saves an image of your screen to the clipboard. Then open a graphics editor and press paste. Your snapshot will appear. I created the image below by first converting it to a grayscale image and then dropping the palette to 16 colours. As you can see, all the text remains readable.
If you see any text disappearing after this conversion you should change either the text colour or the background colour to make sure the text remains legible.