Due to the variations in cameras, lighting, etc. I don't think you'll get anything close to an accurate reading (at least with any that I've looked at). What they may be good at is giving you relative readings (comparing one color to another). I guess they could also get you in the ballpark, but then you'll still have to match by eye.

In any case, you really need to paint the entire colored area. One, it's almost impossible to get a perfect paint match. Two, the new paint will fade at a different rate than the old paint.