Final answer:
The color of a medium using peptones as an energy source is not specified in the context of citrate utilization. Citrate utilization involves a color change from green to blue due to citric acid metabolism in bacteria. The exact color when peptones are metabolized depends on the pH indicator used, which is not provided in this context.
Step-by-step explanation:
The color of a medium when peptones (amino acids) are used as an energy source does not have a standard color change like in the case of citrate utilization. However, the question may be referencing a pH indicator change due to metabolic activity. Citrate utilization involves bacteria that can use citric acid as a sole carbon source, and when they metabolize citric acid, carbon dioxide is released, leading to an alkaline environment. This results in the pH indicator bromothymol blue changing color from green to blue. This specific medium and test are for citrate utilization, not for the metabolism of peptones. If peptones were being used as an energy source in a medium with a pH indicator like bromothymol blue, the color change would instead depend on whether the medium's pH became more acidic or more alkaline as a result of amino acid metabolism, but this typically does not cause a color change to blue, as is seen with citrate utilization. As the question does not provide sufficient context or specify which indicator is used, it is not possible to determine the exact color change. Therefore, the answer is not determinable from the information given.