Take a look at the blue circle below and find the picture inside. I promise you it's there.
Consider this - the average color matching computer can see hundreds of colors (256 to be exact, plus their various shades) but your eye can see over 10,000,000. When you add other variables like sheen and texture you can understand why a 'simple touch-up' can actually be much more. If you don't have the original paint used to paint your walls left over the chances are pretty great it won't match perfectly. Even then environmental factors may make an exact match impossible
The moral of the story is, if you really want to 'touch up' a wall AND you want even color, sheen and tone, the HappyPainters recommend you consider painting the 'bad wall' from corner to corner - even if the color or sheen is 'off' a tad the corner will 'hide' it from your eye. ... See MoreSee Less