Final answer:
Cool-season grasses change colour to yellow, brown, or tan when they go dormant because they produce less chlorophyll, conserving energy until favourable conditions return.
Step-by-step explanation:
When cool-season grasses go dormant, the colour of the grass changes as the plant enters a state of dormancy. During this period, the plant conserves energy and ceases active growth. The change in color happens because the grass produces less chlorophyll, which is responsible for the green color in plants. Without the green pigment being dominant, the grass can appear yellow, brown, or tan, indicating it is in a state of dormancy, waiting for more favourable conditions such as increased sunlight to return to active growth.