The question is quite unclear but I'll try to say a few words that may help you.
You seem to be considering evolution as an individual that decides what is good and what is not. You seem to be considering the evolution as determing at any time point what will be good for a given species. That sounds very wrong.
Evolution is much more than Natural Selection (NS) but for the purpose of this answer I will talk only about NS. NS is a differential of fitness. Fitness is a function of both the expected reproductive success and expected survival of an individual. The environment is part of what determines the fitness of an individual.
So the question, you might be willing to ask is why is having white fur advantageous for a polar bear. The answer is on this post. In short, polar bear live in a very cold climate and their white fur is actually made of clear tubule which let the sun get through and reach their black skin. If it is unclear for you why dark matter warms up faster than lighter matter, you might want to follow a short physics lecture on light.
I would recommend that you have a look at a very introductory class to evolutionary biology such as Understanding Evolution by UC Berkeley for example. It is pretty short (much shorter than reading the book you're reading and you will learn so much more about evolution).
I will not be able to explain you the basics of evolution theory entirely as it would not fit in a single post. So, I am reacting to your first comment but for the follow-up questions, please just have a look at an introductory source of information first and we will all gain lots of time. (No aggressivity here, I just want to avoid spending my day on this single post). Once you've finished an introductory course you are very welcome to ask further questions. Btw, evolutionary biology is a very big field. There are a large number of researchers (me and other users on this website included) who work during their whole to further our understanding of evolutionary processes.
Natural selection must wait for these painstakingly slow changes to provide a large variety of options for it to choose from
This is wrong for two reasons. 1) NS tends to be a relatively fast process for rising the frequency of beneficial mutations. 2) NS does not give options to chose from. It is even the opposite NS remove options to keep only some of them. Mutations create options. In more correct terms, mutation increase the genetic variance while NS (and genetic drift that I haven't talked about yet) remove variation.
A1 allele is a very simple software for students in evolutionary biology that help the to simulate the action of NS and genetic drift on an already polymorphic locus. You might not understand what is meant by "polymorphic locus" but this just mean you need to have a look at an introductory course to evolutionary biology.
if random changes in the cellular chemistry can suddenly alter the colour of hair from black to white, why cannot they change the colour of the hair from black to blue or red or crimson or violet or green, how did cellular chemistry know that what was needed in the arctic climate was only white?
Well, again the hairs aren't really white!
Mutation are random (although this statement can be misleading because there is variation in mutation rate). A mutation is just a "mistake" done during the replication of DNA. An organism does not make mutations to achieve a particular goal. It just make mutations. This is why the vast majority of mutations are deleterious (=decrease the fitness of the individual carrying it).
Now, some mutational effect (in terms of phenotype and not fitness) are more likely than others. For example, a polar bear may already produce a number of pigments that it could "easily" use for colouring its hairs. For example, a polar bear already produces dark pigments (such as melanin) and it is reltaively likely for that a single mutation could darken the fur of the polar bear. If having darker fur was beneficial then this mutation would increase in frequency in the population(assuming no genetic drift which is wrong but this is a story for another time). On the other hand, a polar bear does not produce violet pigments (to my knowledge). As a consequence, it would take a much larger number of mutation (each of which aren't necessarily beneficial) for a population of polar to have a light violet fur (and we would need to assume that having a light violet fur is beneficial).