Here are some examples:
Ασπρόμαυρα εφέ Black and white effects, Ασπρόμαυρα TV, Ο κόσμος δεν είναι ασπρόμαυρος The world is not just black and white...
In Greek, we would almost always use the compound word: Ασπρόμαυρα. You're welcome and your questions are always welcome.
In English 'black and white' is a so-called Irreversible binomial (see https://en.wikipedia.org/wiki/Irreversible_binomial). Is this also the case in Greek, i.e. one would never say μαυρόασπρο?
Sometimes I don't 'get' Duolingo. It's marked wrong a sentence where I got everything right apart from leaving a space between 'ασπρό' and 'μαύρο'. That's OK. But other times, I make typos where I leave out letters that actually change the whole word, and it marks it correct (like 'αλοο' instead of ''αλογο' - Ι have a very stiff keyboard which sometimes doesn't 'take'). Just saying.
Duo never rejects translations for spaces, (unless you have two words together like this "the dogran to the boy" which doesn't mean anything). So, your sentence was rejected for another reason.
Follow these tips to make learning easier and with few mistakes:
TIPS TO MAKE LEARNING EASIER + HOW TO REPORT A PROBLEM https://forum.duolingo.com/comment/22424028 And check out the Greek Forum here with more links. https://forum.duolingo.com/topic/936 If you have any questions just ask.
We, moderators, have no control over the grading algorithm. We write the sentences but the grading is done by a computer program. Personally, I would say that there is a definite vocal difference between "απρπόμαυρο" and "άσπρο μαύρο". But as I said that is all regulated by Duolingo Administration so you might want to report it: https://support.duolingo.com/hc/en-us