But if you were talking about a specific green out of several green ones, wouldn't you say "That green is a pretty color"..?
"The green is a pretty color"... The sentence structure is just very awkward there and sounds like bad English. "The green one is pretty," is more acceptable. Or "that is a prettier green", if we continue with your scenario.. but that would change the French sentence completely as well.
I disagree that it's an awkward structure, or, as Wunel suggests, a dated or archaic form.
If I was looking at a dress which incorporated a nice shade of green, I could say "the green on this dress is a pretty colour", but sometimes we can omit unnecessary information, so I would say "the green is a pretty colour" because whoever I am speaking to already knows I am talking about "this" dress.
I'm inclined to agree with you here, "the green is a pretty colour" when referring to clothes would be an awkward one to listen to. However it is the kind of syntax you might find if you went back to the mid 20th century and more specifically to the upper classes and pseudo-nobility, so it can be regarded as correct, if a little dated.
The meaning of the sentence is that the color green is a nice color, and if it was worded that way you would use "the". "Green" on its own is used as a noun here. While we use articles a lot with nouns in English, we usually do not do it with colors, especially in general statements like this.
E.g. --Pink is the new black.
The pink is the new black---- just doesn't work.
As an addendum of sorts, I think what themathkid was asking is why does the sentence not say "the green" as in he wants to know where the "le" disappeared to.
To hopefully answer his question, then: In English the use of articles, both definite and general, is not compulsory and in most situations is not really necessary in order for the listener to understand you. In French, however, use of articles is compulsory. So when translating from English to French expect to use a lot of le's and la's where you would not be using "the" and when translating from French to English expect to have a lot of le's and la's disappear.
It should, but it is probably for the same reason why none of these would: "elegant", "lovely", "nice-looking", "fetching", etc... I haven't tried it myself, but I would guess that Duo's engines might even have a problem with "attractive".. There are too many synonyms out there. Keep it simple, i.e.: joli/jolie=nice; joli=handsome; jolie= pretty.
How does a feminine noun look? Do you mean that it doesn't end in an 'e'? It's true that many feminine nouns end in an 'e' but it's no guarantee. In fact -eur is one of the common feminine endings (with the exception of words for many professions and mechanical/scientific things).