Natural Selection proposes that organisms that are better adapted to their environment are able to survive and reproduce.
Even though these frogs are the same species, their shades of green is what helps them to survive in their habitat. In this case, frogs with a lighter shade of green are able to be seen by predators easier, whereas frogs with darker shades are able to blend in (camouflage) more with their surroundings. After a period of time, due to them being easily seen, lighter-shaded green frogs will die off.
To anseer your question, natural selection would have a gradual affect on the frequency of the alleles. Lighter-green allele frequencies would eventually cut off, and darker-green allele frequencies will increase.
Hopefully the following image will help:
As seen in the image, (please forgive the quality, as I had drawn this on some random kids drawing site on the internet...) you can see the affects of natural selection on the allele frequencies. The brighter-green shades gradually decrease over time, as the darker shades increase.
Hope you find this helpful.
