Strips of pH paper are much cheaper and easier to use than a pH meter. They give you a quick way to estimate the pH of a solution without any expensive equipment or pre-calibration. They have limitations, however. Remember, there is a great deal of uncertainty in a measurement when you use these strips.
Sometimes, it is difficult to match the color of the strip with any one of the colors shown on the box. There can be times when the pH paper looks like it is green, but perhaps not quite the same shade as what the box shows. In cases like these, it's difficult to know whether you are reading the pH strip properly. If you're red-green colorblind, of course, pH paper becomes very difficult to use, because you won't be able to tell the difference between these colors.
The colors of a pH strip correspond to pH ranges, not to a specific pH; usually, they read in increments of 0.5. Consequently, when you use pH paper, you can't get an exact number. If you need a definite number for the work you're doing, pH paper won't be very helpful. You can estimate the pH, but you'll have a high uncertainty value; pH meters, by contrast, will give you a more exact figure.
The pH paper is not temperature-compensated, which may introduce inaccuracies, if you're working at very high or low temperatures. A pH of seven, for example, is neutral at room temperature. But, at your body temperature, neutral pH would be about 6.8. (The pH of your blood, by the way, is not actually neutral -- it's slightly alkaline.) The pH paper, however, does not compensate for this change. Not all pH meters do (although some do).
High or Low pH
At very high or low pH values, pH paper may not give an accurate reading. If the pH is below 0, for example, your pH paper would not give you an accurate reading, since pH strips are not designed for extreme pH values. In fairness, of course, you'll seldom need to measure solutions of strong, concentrated acid or base; if you have fuming sulfuric acid, you probably already know the pH is going to be very low. Nonetheless, this, too, is a possible source of error to bear in mind.