Final answer:
M9 paper changes color to indicate chemical contamination, resulting in dark clumps against a blue-green background for a positive reaction, and a greenish-brown background with possibly a graining appearance for a negative result.
Step-by-step explanation:
M9 paper, which is used for detecting chemical contamination, changes color when it comes in contact with certain substances. A positive reaction will present moderate-to-large sized dark clumps against a blue-green background, distributed uniformly over the surface of the test circle. On the other hand, a negative result shows no clumping and may have a slightly graining appearance against a greenish-brown background. This reaction pattern is different from the color change exhibited by pH paper, which contains a set of indicators that change color at different pH levels to approximate the pH of a solution.