I reported it because it should be accepted. It is true this could mean something other than people turned red, in the event you were referring to people, where I'm from you would never say they turned red (in the USA)! I contend "They blushed" is the best assumption for my region, even though not as literal.
In UK English, normally you go red, or in this case, they went red - but it is marked wrong. To turn red happens to tomatoes or cherries or whatever else changes colour as it ripens (because they don't go back to being green, but people normally change back after they flush... turning a colour normally implies permanence in UK English).
Duolingo usually accepts UK spelling, with some exceptions (i.e. Brasil) but often disregards phrases that are different from American English. It feels as though those writing the courses have a strong American inclination. I would report it, but I doubt they would bother to add additional answers.