Generally photosynthesis refers to utilising photons of light to convert it into chemical energy..The photons of light is absorbed by the pigment called chlorophyll which is of two types: Chlorophyll A and Chlorophyll B. Chlorophyll A absorbs a lot of red and some blue light whereas chlorophyll B absorbs a lot of blue and some red.. Neither absorbs green which is why the plants are green in colour and hence the rate of photosynthesis is less or minimum in green light.
Also in other words, both chlorophyll A and B together absorb blue and red color more. So rate of photo synthesis is more in blue than green.
Hope you understand!