Final answer:
A bright yellow color observed after adding phenol red to pool water indicates that the pH is low and the water is acidic. Adjustments like adding sodium carbonate would be necessary to raise the pH to an optimal level.
Step-by-step explanation:
When a pool operator uses phenol red test reagent to check the pH of the pool water, observing a bright yellow color indicates that the pH of the pool water is low. Phenol red is a pH indicator that turns yellow at a pH level below 6.8, which signifies that the water is acidic. The ideal pH for pool water is around 7.2, so a yellow color suggests that adjustment is necessary to prevent discomfort or harm to swimmers, possibly through adding a base such as sodium carbonate to raise the pH.