Definitely! vowel + S + vowel = "S" pronounced "Z". Sure, as usual in French, there are exceptions, as "bisexuel" which is pronounced [biSSexu-el], but here it's because it comes from "bi" (2) and "sexuel". But no doubt about this case, feminine "grey" is pronounced "griZ" (written "grise(s)" of course).
I could be wrong but I believe it is because the color marron is taken from a real life object (a chestnut) and therefore is an exception to the rule about maintaining agreement with the rest of the sentence.
Grise as far as I know is not taken from a real life object and thus is subject to all the normal rules.
For inexplicable reasons, most colours are "invariable" and so are not pluralized.
"French color adjectives derived from nouns, such as animals, flowers, fruits, gems, and metals, are usually invariable:" - http://french.about.com/od/grammar/a/adjectives_inv.htm
Yes, I get it. So 'Orange' is another example, although as you pointed out 'Rose' is not the indefinite form. I found a long list of colours in their plural and indefinate form here - http://french.about.com/od/grammar/a/adjectives_inv.htm.
The French word for Orange is 'Orange'. The French word for white is 'Blanc'. The plural masculine of white is 'blancs', following the normal structure. But the plural masculine of orange is still 'orange'. In fact all the conjugations of orange will be 'orange'. It is in a group of colours that will always be in the indefinite form - no matter if male or female, singular or plural - because its name derives from the same French word for an animal, metal, fruit, jem or flower. E.g. Cerise is the French word for "Cherry" and also for the colour "cherry red". Whilst some are more obvious, as a learner like yourself, you'll just have to learn them as you come across them.
Well, what is the purpose of this type of question? It is to understand the audible differences. I'm particularly good at writing comprehension, but quite weak on audible comprehension as it takes me a long time to process the sounds. That is exactly the point of this exercise. If you cannot either get the subtle differences between similar spoken words, or work out the conjugation from the context, then you're not truly learning the language in my opinion. To answer your specific question, I don't know if there is such an option on the PC version, but you can skip the question. However, if its difficult to understand - assuming its not wrong or incorrectly pronounced, which you can report to Duolingo - its an opportunity to learn.
I find the actual recording to be poor and difficult to understand. And it is so strict on spelling that most of the time I will get what they are saying but not spell it correctly and have the question marked wrong. Spelling comes with time, and I don't find this system helps me learn at all.
"...I will get what they are saying but not spell it correctly and have the question marked wrong"
But are you learning anything? Duo is not about "levelling up". That is an incentive to learn. If there is a problem with the audio, use the function on the page to report it or on the left <--
Re spelling, you are after all using a website where the main interface is to type. The premise of Duolingo is to teach the language using the website as a medium. If you want to concentrate on oral learning only then take a class.
If you want to get the best out of Duo, then perhaps treat each exercise like a lesson. When we have a real lesson we carry a dictionary for reference and a grammar book. You can use Google Translate as your online interactive dictionary during an exercise and this forum for grammar explanations. When you put it all together you should get a lot more from the site.
It's telling you that a word spelled g-r-i-s can be "(you) are exhilarating." That's because there is a verb, griser, that conjugates to grises. But that's a different word than the adjective gris/grise. Duo shows you some of what something can mean as shown; it's not necessarily appropriate for the sentence, and could be the definition of a homograph, as in this case.