Translation:I do not want green pants.
Depends on what you're used to saying in your corner of the world. Pants sounds unambiguous to someone who's never heard it used to describe underwear.
Duo's translations seem to follow American English, with words from other English-speaking countries added as alternative answers as they are reported.
色【いろ】 means colour, and many names of colours come from appending 色 to the names of things which are that colour. When it comes to ピンク that's a loanword which already refers to a colour, so adding 色 would be redundant (but it does happen rarely).
With 紫色【むらさきいろ】 the 紫 refers to a type of plant whose roots can be used to produce a purple dye.
With 緑色【みどりいろ】 the 緑 is a word for freshly grown foliage.
オレンジ refers to the fruit, オレンジ色 refers to the colour.
In many cases you can leave off 色 and be understood, just as people do this in English, referring to colours by naming things.
On its own, 緑【みどり】 refers to fresh foliage, so 緑色 【みどりいろ】 is like saying "the colour of fresh leaves". In this case perhaps especially, you can just say 緑 to mean green though as it gets used to describe a colour more than anything else, and people would know you're not talking about a ghillie suit.