Nigel Wilcockson (who works at Penguin) says:

Brief potted answer goes something like this (and this is off the top of my
head, so do please let me know if the facts need to be accurate!):

Penguin Classics first appeared in 1946, when they had typographical covers
and coloured borders (colour varied from cover to cover). They acquired
'historically apt' images and black livery in 1963. The look was updated in
1986 at which point other series were rolled into Black Classics (notably
Penguin English Library which dated back (I think) to 1965 and which had
orange spines).

As for modern/20th-century Classics, As far as I know, the series dates back
to 1961, when the livery was grey. I don't know exactly when the eau-de-nil
came in, but it must have been late 1980s, perhaps 1989, and my guess is
that the series was massively increased in size at that point. Now has a
silver livery.

Penguin orange dates back to 1935 when it was used for novels. (At that
time, there was a simple colour-coding system in place, so that, for
example, green = crime, blue = biography.)

I'd recommend a book called Fifty Penguin Years, published in 1985.

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