Svart and rosa are a bit special.
Svart does not change in singular indefinite form, but gains the regular -a in plural or definite. (En svart bil, ett svart hus, svarta katter; den svarta bilen, det svarta huset, de svarta katterna)
Rosa never changes, it's always "rosa".
That only goes for definite nouns, but here the noun is indefinite. Not handsken but en handske.
The articles and adjective still agree, but instead it becomes:
en svart handske 'a black glove'
en stor handske 'a big glove'
ett svart hus 'a black house'
ett stort hus 'a big house'
(svart happens to be the same for en and ett words in the singular indefinite, so I added stor/stort which has different forms to show the difference).
Hey, don't know if it's useful to warn here, but I had this sentence has a "type what you hear" exercise but when you have to click on the words rather than type them yourself (first levels of the course). I got it correct with the swedish words but DL told me it was wrong and that the correct answer was "a black glove", so basically it wanted me to translate but I only had the choice of using the swedish words. ^^' I reported the bug so there's probably nothing more to do now, but I wanted to be thorough :) Thanks for all the hard work you're doing on the swedish course!
No, you can speak about one glove, 'en handske'. It may be more common to speak about a pair of gloves 'ett par handskar' or, say, my gloves, 'mina handskar'. But if you've lost your left glove or there's a hole in your right glove, there is nothing strange in speaking about just one glove.