In the usage I am familiar with, red originally referred to blood, but in this context it loosely means fields of biology focused below the organism level that are often primarily or exclusively studied in labs or perhaps hospitals and the like. So, biochemistry, molecular biology, cellular biology, virology, immunology, genetics, pharmacology, neuroscience, and so on.
Green referred to actual natural environments and in this context loosely means fields focused on the organism level or higher and that might involve field studies, or possibly farms or so on. So, zoology, botany, entomology, ecology, forestry, wildlife management, animal science, various agricultural and food sciences, and so on.