"There are gray and purple ducks."
Translation:Il existe des canards gris et violets.
Good question. There are trees in the forest, but if we cannot see or hear them do they exist?. Il y a des canards gris et violets invites you to look across the lake and inspect the little darlings; il existe des canards is an existential statement - these strange birds really do exist, somewhere. At least, that's how it appears to me. "Il existe" is firmer, more definite.
That's my opinion, but I am not certain. If someone wants to jump in and offer another view or a clearer distinction, I'm all ears.
I think "Il existe" is one of those idiomatic phrases that doesn't change no matter what you are talking about. What exists is a universal singular fact (in this case that there are grey and purple ducks). To be sure, you could check with one of the moderators if you come across them on another thread.
Most colors change to agree with the gender and number of the noun they modify, but as with most rules there are exceptions. In this case, "gris" ends in an "s", so it doesn't change for the plural. Here is a chart with a lot of the colors and their forms for different noun forms: https://french.kwiziq.com/revision/grammar/how-colour-descriptions-adjectives-change-according-to-gender-and-number. Some other exceptions for colors are listed below the chart, and there is also the exception described by ripcurlgirl above of two adjectives designating a single color.
Let's assume this is the first time you've used either "gris" or "violet". You apparently think "violet" is grey and "gris" is purple. Before you dismiss this as a silly idea, there are many false friends between French and English.
How can the program tell the difference between a mistake and an imprecise answer? What if you'd said "La pomme mange la fille" for "The girl is eating the apple"? Should that be accepted as well?
Yes, I know, all the information is there and, technically correct, but why did you change the order of the adjectives? Perhaps gris is the dominant colour? Such as the Australian galah - they are predominantly grey with a pink head. It is far simpler to translate the sentence as it is given to you. It is almost as if you are going out of your way to find fault.
If you are asked to translate a sentence rather than interpret it, the order of the words is very relevant. There are times when a direct translation between two languages doesn't work. This isn't one of them. The computer has to look for the direct translation which has been programmed into it.
In reading the French sentence you would be surprised if the object in question was almost entirely purple with hardly any grey. The order of colors in a sentence creates an expectation that the first one is at least as significant as the second one. It is natural to place adjectives and adverbs in a descending or ascending order. Because of that, listeners and readers expect that to be the case. It is not a grammar rule. It is a human communication rule in the Western world. In English, the common practice is descending order of significance.
Some people find it useful to break common patterns and generate a small amount of broken expectations. President Trump's speaking style uses simple words but an erratic flow of those words. The result is you have to pay attention to what he says to follow its direction. That way he gets to repeat his points over and over while the audience focuses closely on what he is saying. Love him or hate him everything he says gets inside people's head. He lives there rent free. What he says is virtually all anybody talks about in the North American news. However, the Duo computer would blow its circuits trying to translate him.
You emulate President Trump's inconsistent word order style at your peril. The Duo computer won't accept it ever. And if you are at this level in French studies, no one will understand you when speaking it.
Over at https://www.thoughtco.com/invariable-french-adjectives-1368796 under Multiple Colors they state, "2) If each item has all of the colors, the adjectives are invariable des drapeaux bleu, blanc, rouge - red, white, and blue flags (e.g., French flags) des chapeaux rouge et noir - red and black hats" If this is correct and we are talking about ducks that have both gray feathers and purple feathers, gris et violet would be correct. But if we want to say there are gray ducks and there are purple ducks, then gris et violets would be correct. Am I reading the rule correctly?
Male Mallard ducks (which are the most common wild type in North America) can reasonably be described as having both grey and purple colors. Lot of green as well but that isn't mentioned in this sentence.
In my experience I have never seen a male wild duck that didn't have those colors. Quite noticeably actually. I assume there are other types but I haven't seen them personally.
I have seen a lot of uniformly white, domestic breeds of duck, if that helps.