I think I'd have trouble calling or “equally valid.” Logically, in lists conjoined with or, not all the items in the list need apply, but only one or more. So:
In my school, our teacher used various colors of chalk, such as blue, green, and purple.
implies that the teacher used various colors of chalk, including blue, green, and purple (possibly others, too, but at least those three were definitely used), while:
In my school, our teacher used various colors of chalk, such as blue, green, or purple.
implies various colors were used, like blue, green, or purple (but in fact, purple may never have been used – you merely remember that those kinds of colors would be used by the teacher).
Having said all of that, we're talking about a subtle nuance that I might not even pick up on, not I unless I spent some time analyzing the conjunction carefully (and, to be quite honest, in a sentence like this, I probably wouldn't spend much time analyzing the conjunction carefully). At face value, I'd probably interpret either sentence to mean:
In my school, our teacher used various colors of chalk, such as blue, green, purple, etc.
I highlighted the word various, because it's a key word in this sentence. If the subsequent list is intended to specify the exact colors of chalk that you remember your teacher using, then and would probably be the better word, especially if you really remember your teacher using all three of those colors individually. However, if you're merely giving examples of the kinds of bright colors you recall seeing on the board, without regard to utter accuracy, then or might be a better word.
So, I'd say that whether or not the conjunctions are virtually interchangeable depends on context; specifically, how important it is that the sentence be precise? Everyday conversation often doesn't require such precision. For example, when my children tell stories, they have a tendency to interrupt each other and quibble about minor details. Such a conversation might go something like this:
Beth: When we were in fifth grade, our teacher used various colors of chalk, like blue, green, and purple.
Seth: No, Mr. Jones never used purple chalk.
Beth: Yes he did!
Seth: [shaking his head adamantly] No, he used red chalk sometimes, but never purple...
It drives me crazy when they do that! I'll often interrupt the interrupter, and ask, “Is it really all that important whether or not the teacher used red or purple chalk?”
However, if the sentence is more technical in nature, perhaps the author (or speaker) should pay more attention to conjunctions. Consider:
Domesticated jerboas have been known to live for months eating nothing but fruit (e.g., apples, pears, oranges, and bananas).
Domesticated jerboas have been known to live for months eating nothing but fruit (e.g., apples, pears, oranges, or bananas).
I would interpret the first to mean that the jerboas were eating a variety of fruit, while the second might imply that you could pick any one of the four fruits, and feed that to your domesticated jerboa for an extended period of time. Since there isn't very much literature about domesticated jerboas, it's important that the author get this right! Perhaps it would be better to restructure the sentence in a way that eliminates any possible ambiguity.
I thought of a couple more examples:
A myriad of games will be played at the Olympics, like gymnastics, swimming, track and field, cycling, and basketball.
A myriad of games will be played at the Olympics, like gymnastics, swimming, track and field, cycling, or basketball.
In this case, or just sounds wrong. All of those sports (and others) will indeed be played at the summer games; it's not like the IOC will decide at the last minute to keep boxing but leave out cycling. On the other hand:
We could plan several events for this year's company picnic, like horseshoes, face painting, sack races, or volleyball.
sounds better than:
We could plan several events for this year's company picnic, like horseshoes, face painting, sack races, and volleyball.
because or implies that you're brainstorming; not every event need to be included for your idea to sound good.
I guess my bottom line answer is that the words mean slightly different things, and should be selected appropriately. NOAD says:
and (conj.) used to connect words that are to be taken jointly : bread and butter
or (conj.) used to link alternatives : a cup of tea or coffee
However, there are contexts where the either one could be used with little loss of meaning (such as remembering what color chalk Mr. Jones used in fifth grade).