Genetic bottle neck
Explanation:
Genetic bottle neck is defined as an evolutionary event during which a population is drastically reduced to to a significantly smaller size within a short period of time.
A genetic bottleneck can result from various events such as natural disaster, hunting of a species to a point of extinction or habitat destruction to cause the death of large number of organism.
In the given situation a typhoon which is a type of natural disaster caused the population to reduce to a size of 20 individuals. Then we find that the allele frequencies were greatly changed and the allele for colorblindness became established.Hence, a large population of present day inhabitant are color blind.
So we can say that the population experienced a genetic bottle neck.