Question on how Evolution works to produce new characteristics

No. In that scenario, it would be "differential reproduction" that would change the average colour scheme of the frogs over time.What that means is that those with better camouflage in relation to the new plant environment have a better chance of surviving their predators, and are therefore likelier to live longer than the average.Living longer means a better chance of producing more offspring, so the "better camouflage" characteristics will become more common in the population over generations.The time it takes for the population to be as well camouflaged as they were before the environmental change will depend mainly on three factors. These are (1) what (if any) chance variations happened to exist in the population at the time of change that suit the new environment; (2) how soon individuals receive new mutations that happen to give them a better colouring - another chance element - and (3) how important camouflage is to their survival (the more important, the faster the new characteristics become fixed over the population).Hope that helps, and welcome to the board.

Because the variation in colouring isn't produced according to need, but by chance. Yes. Although chance favourable mutations could occur in more than one individual over the time involved.