I'm not a native speaker, so I may say something incorrect.
On the link I gave you, at the very top, in grey, (ironic) it says inflections (adj) of gris. Here is my understanding:
Gris - masculine singular
Grise - feminine singular
Gris - masculine plural
Grises - feminine plural
As you can see, singular and plural (masculine) are the same. For violet, it is:
Violet - masculine singular
Violette - feminine singular
Violetts - masculine, plural
Violettes - feminine, plural
Now, don't ask me why, because I don't make the rules of French and if I did they would make sense. Perhaps a native could reveal that mystery.
Because the il is not referring to the plural canards. It is a "dummy" subject for the impersonal verb existe. As when we talk about the weather: it is snowing, freezing, raining, or it is hot/cold. There is no "it". The reason we use "it" is that grammar will not allow us to construct a sentence without a subject, so we use "it" to cover our psychological need. We could say "Snow is falling", "The weather is hot" but we don't. English uses this impersonal "it" all the time. "It was van Gogh who painted that picture" What does "it" mean there? Nothing. It is a place holder. We could just as easily say "Van Gogh painted that picture".
Now, to your main point about why not plural "ils existent". Consider this sentence. "It was five people I saw, not three". See? You just can't make this plural.
You have an editor's eye! I wondered the same thing. I'm curious whether French uses commas to separate coordinate adjectives the way we do in English: There are gray, purple ducks would definitively mean "ducks that are gray and purple." But "There are gray and purple ducks" is decidedly ambiguous.
Because in English, at least with this grammatical structure, you have to use the plural verb for the plural noun, so "there exists a duck" but "there exist ducks".
We're told that where there are multiple colour adjectives they all take the singular masculine form, eg "elle a une robe noir et blanc", yet all of a sudden I'm marked wrong for "des canards gris et violet" and told I should have written "des canards gris et violets". I suppose the latter means "grey ducks and purple ducks", rather than "grey+purple ducks", but the English sentence can mean either, so I think both translations should be allowed.
A thorny issue, a Dhroigeann, a chara.
In my view this is about the difference between gris et violets (grey and purple) and gris violet (purplish grey or greyish purple). Perhaps this makes the Oxford comma reference elsewhere in these comments redundant? "Gris et violets" could mean a mixture of ducks, some of which a grey and some purple. It could conceivably mean ducks that have blocks of grey and blocks of purple on the same bird. "Gris violets" on the other hand must mean ducks of a uniform colour that we could call a greyish purple or a purplish grey.
That's my take on it, but I'm more than happy to be corrected.
Go raibh maith agat, but my problem is that this page https://french.kwiziq.com/revision/grammar/compound-colour-names-and-colour-names-derived-from-things-are-invariable (by "a professional French teacher, translator and linguist") tells me that "a black and white dress" counts as a compound colour adjective, so it keeps the masculine form, "une robe noir et blanc" - surely it's not a dress of a uniform whiteish black or blackish white colour, but a dress with blocks of white and black, so I would expect ducks with blocks of grey and purple to similarly keep the singular form, "des canards gris et violet". I don't say I'm right, I say if I'm wrong I don't understand why.
I may be wrong but it seems to me that the English sentences are derived from the non-English originals and this can lead to ambiguities in the English sentence that were not in the source sentence.
Anyway, in this particular case, I think the French is referring to two distinct sets of ducks of different colours. At least, according to this advice about agreement of adjectives of colour on ac-versailles.fr
dictée n°2 : 3ème1
c) Il faut faire également attention au sens que l'on veut donner à la phrase car ça fera varier l'orthographe des adjectifs qualificatifs
ex : des chats
blanc et noir(chaque chat est blanc et noir), des chats
blancs et noirs(il y a à la fois des chats noirs et des chats blancs).
Thanks a lot, that confirms my initial idea. Interesting reference generally by the way, will have to look at other things it says as well!
As I see it, the rule you refer to doesn't apply here because we are not dealing with a compound adjective.
This sentence is, it seems, talking about TWO groups of ducks, one with grey plumage, the other with purple. i.e. "grey ducks and purple ducks" as opposed to "grey-purple ducks".
I repeat here a quotation from my post from a month ago:
des chats blanc et noir (chaque chat est blanc et noir), des chats blancs et noirs (il y a à la fois des chats noirs et des chats blancs).