Hydrogencarbonate indicator can detect increases and decreases in carbon dioxide concentration.
It is normally red. However, an increase in carbon dioxide changes the indicator to yellow and a decrease in carbon dioxide changes it to purple.
Photosynthesis only happens in the light. Respiration happens all the time.
Both photosynthesis and respiration are occurring but the rate of photosynthesis is higher than the rate of respiration, therefore the net gas exchange happening is carbon dioxide into and oxygen out of the leaf.
The removal of carbon dioxide from the hydrogencarbonate indicator into the leaf would change the indicator from red to purple.
When light intensity is low the rates of photosynthesis and respiration are equal. This is known as the compensation point and there is no net gas exchange.
No net gas exchange would cause the hydrogencarbonate to remain red because no change in carbon dioxide concentration will be detected.
Only respiration is occurring as there is no light for photosynthesis, therefore the only gas exchange happening is oxygen into and carbon dioxide out of the leaf.
The release of carbon dioxide from the leaf into the hydrogencarbonate indicator changes the indicator from red to yellow.