I would like to know if an 'and' is always necessary before the last item in a colon list.
Is the following correct, or should I place an 'and' before 'grapes'?
The basket contained a variety of fruits: bananas, apples, oranges, grapes.
The deceptively simple answer is that and is customary before the last item in a comma-delimited list
(in a sentence, at least; bulleted lists on slides don't fall under grammatical rules).
This means that leaving it out draws attention to the sentence.
And that's exactly what the author intends to do -- draw attention to the sentence.
The result of leaving the and out, in speech, is to end the sentence without a full-stop intonation dip. That leaves the perception that something is still left to come, and we strain to hear it -- or at least our instincts do, and our parsing habits are guided by them. The result is a peculiar form of emphasis.
You should put "and" before the final item in the list. According to ThePunctuationGuide.com,
The colon is used to introduce a list of items.
The bookstore specializes in three subjects: art, architecture, and graphic design.
Do not, however, use a colon when the listed items are incorporated into the flow of the sentence.
The bookstore specializes in art, architecture, and graphic design.
The bookstore specializes in: art, architecture, and graphic design.