"Is this your new, black coat?" is a confusing use of a comma, the proper English grammar is "Is this your new black coat?" Commas are to be avoided with a stack of adjectives in English. For instance I could not say to to my wife "That's your new black dirty mangy ugly unstylish coat that cost too much and makes you look fat." because that would be rude, but it is good grammar nonetheless.
Placing commas is not affected by whether adjectives are stacked or not. It is the nature of the modifiers that determine the use of commas.
To simplify the rule about commas, just remember, if it maintains the sense of the sentence to place and in between stacked adjectives, use a comma. In the example offered above: new (and) black (and) dirty(and) ..etc. all indicate that they are adjectives modifying the noun coat and therefore -require- a comma to separate them.
The problem comes up when there are adverbs that appear to be adjectives in the sentence. Adverbs are not separated from adjectives by commas because they modify the adjective. In fact, the absence of commas is a good way to recognize which words are adverbs and which words are adjectives in a sentence.
Eg: a dark, blue car is a car which is both dark and blue. Dark and blue are both adjectives which modify the noun car. The car is dark (maybe with tinted windows, no chrome, in the shade etc.) and some shade of blue (perhaps a very light shade). We know this because the presence of the comma tells us they are adjectives which means they must modify the noun.
Eg: a dark blue car is a car which has a dark blue paint job but may otherwise be quite bright (lots of chrome, lots of open windows including a sun roof to let in light etc). We know this because the absence of a comma indicates that dark is an adverb which must modify the adjective blue.
Placing or not placing a comma in a string of modifiers changes the meaning of the sentence.
All this is about English not French, but there are non-native English speakers on this board who may be misled by the simple statement that commas are not needed in a string of adjectives.
That's a reasoned response. Yet, it establishes two English grammars. Written and spoken. There are not such two things, excepting only in legal documents where the difference between a comma, a semi-colon or the absence of either can indeed significantly change the meaning of a law. In spoken English a comma is suggested by a verbal pause, or by the common rules of grammar that tell a writer of English that a comma must be used. For example in a list of nouns three of longer, the rule is comma after each noun except the last, and between the next-to-last and the last where an "and" is used.
You give a great example. What's is the difference between "dirty, dark, blue car" and "dirty, dark blue, car"? Well, in the second, which is describing a "dark blue" color, the "blue" is no longer a adjective, it is in use as a noun. Adjectives can be stacked without commas. But an adjective can change into a noun in the usage, and then we use a comma when writing and some verbal trick -- emphasis, modulation, tone or pause to emphasize the noun-ness of an adjective.
bvanw, I disagree with you about "dirty, dark blue, car".
I would say that in that phrase the "blue" remains an adjective because it is used to describe the noun "car".
The "dark" is now an adverb, because in the phrase it is modifying the adjective "blue" rather than the noun "car".
Adverbs also modify adjectives and other adverbs. If you do not believe me then why not do an internet search for "adverbs modify adjectives"?
In "dirty, dark blue, car" 'blue' is not a noun, because in that phrase it is not signifying "a blue" or "the blue" but giving information about the colour of the car and therefore an adjective.