Translation:The flower in that garden is a perfect color.
"has" and not "is". Also, the most natural is to say just "Το λουλούδι σε εκείνο τον κήπο έχει τέλειο χρώμα. ". We rarely use " ένα", in that circumstance. But even more natural is ""Το λουλούδι σ'αυτόν τον κήπο έχει τέλειο χρώμα. "
It should be accepted already.
From what I can see in the incubator, the last change to this sentence was two months ago, and in its current state "has a perfect colour" is one of the alternatives.
has a perfect colour! The alternative 'has' is not offered, and 'is a perfect colour' is simply mistaken