Translation:She wears a red sweater.
It is pronounced "aka" but the full word is "akai." (赤い) it is just said quickly here.
Aka is read, but when you want to say something is red you add an い to it turning it into an いadj.
singular or plural is not specified in this sentence (like a lot of these sentences) so either one should have been accepted. whichever one they chose is arbitrary
This would depend on the word "a" here. If there is an "a" then there is only one sweater. If there's no "a" then there are more than one. The numbers must match.
She wears red sweaters
She wears a red sweater
This couldn't be translated as "she is wearing a red sweater," right? What would that be called, present-progressive tense, idk?
Right. 'wearing' would require the "-te imasu" conjugation of the verb
"Jumper" is the more natural British English word for セーター. Didn't try it, so it might already be in as a correct alternative.
In NZ English, would use "Jersey" or "Pullover" - neither of those acceptable either.
Can't we turn all the colors into adjectives? Because sometimes they put "紫色/ むらさきいろ の ズボン", so I can't imagine.
What you can do with all colors (using them as no-adjectives):
What you can only do with i-adjective colors (I think around 7 in total):
I think putting 色 after the color is optional. You see it sometimes, but not all the time.
紫色のズボン = purple-colored trousers
紫のズボン = purple trousers
Some colors do have the 色 in them, like 茶色 or 黄色. Those are not optional of course.