"The book is orange, violet, and yellow."
Translation:La libro estas oranĝkolora, violkolora kaj flava.
Purple can also be purpura. Violkolora literally means the color of a violet (the flower).
There isn't a word for pink, so it's rozkolora (the color of a rose). This can be a bit confusing as roses come in many many colors, so just remember the English phrase "rose colored glasses", which specifically means pink.
That's really only the case if there is a possibility of confusion. "Those roses are yellow and pink" Tiuj rozoj estas flavaj kaj rozkoloraj comes to mind, but, "the sunset is all blue and pink!" La sunsubiro estas plene blua kaj roza! would not be an occasion to think that there are blue roses in the sunset.
One of the constituent elements of what makes a rose a rose is it's color. In several European languages that color is expressed as some form of "Roza". In fact, I have no idea where the English word "pink" even came from.
Is this where the ending vowel helps clarify? Oranĝo = an orange (noun); oranĝa = orange-colored (adjective). Although what we call in English an "orange blossom" is not an orange-colored flower, but rather the flower of a an orange tree (again, not an orange-colored tree, but a tree which bears fruits called oranges.) Ending vowel and context, perhaps are the clues that help.
I could see it being a good idea if there would otherwise be ambiguity, but that isn't often the case. Like I said, it is a stylistic choice rather than a grammar rule, and even in English there are styles that forbid it, like AP style. Outside of the US it is almost never used in non academic writing. English is the only source language for Esperanto that doesn't forbid the general usage of serial commas.
It may be ambiguous at times, so why ask people to think whether or not it is ambiguous? Just require it at all times. It keeps it simple. There is no reason this couldn't be the rule in Esperanto. I hate when this comma is dropped. It just looks bad and does have the effect of making me think something is going to come next, just to find a period.
Zamenhof was a bit (perhaps too) lackadaisical in laying out the rules for punctuation in Esperanto. Perhaps because he knew that people would be coming to this language from many different linguistic traditions and mindsets. As I said earlier, if you wish to use it, you may. but someone raised in, say, Arabic or Chinese may not have the same sense of ambiguity that you do. Requiring that a comma be placed at a certain location will, for a considerable fraction of writers, result in too many commas in too many places, and thus intensifying, rather than alleviating, the ambiguity.
I hope that this helps you, and isn't too ambiguous.