How about reading your manual? It would tell you!
If you don't have the book, download an electronic version from the Canon website.

However, consider this for a moment -
I would never use such a feature for the following reasons:

• You are very limited in what you can do.
• The function isn't always reliable or accurate. The camera's idea of "red" might be different to yours. It will often include/exclude areas that you don't want to have included/excluded.
• Imagine if you just happened to take your best photo ever, but instead of having a real color photo, you only have some partial b&w thing that might look totally awful. You'd kick yourself.
• If you do the "Selective or Partial Desaturation" (as it is called correctly) in post processing, you have much more control over it, and will get a way better result. Plus you can keep your original color version, too.

This also applies to any color effects done in camera like b&w or sepia - you're better off NOT to.

If you don't have your own image editor, you can go to www.picnik.com and use their effects menu which makes it very easy.