The Nutrient-TTC agar color is normally light yellow when the agar is cast. After testing, during the incubation phase, the agar may change to a light green color. This color change is a result of either a microbial-induced or chemically-induced pH change in the media. This color change alone does not indicate the presence of microorganisms. Development of red spots or other growth on the agar are an indication of microorganisms.