"A gray egg."
Translation:Een grijs ei.
'Ei' is indeed a 'het'-word, but since the indefinite article 'een' is used here, the -e is not added. So:
- 'een grijs ei'
- 'het grijze ei'
As BeckyCar mentions, the -e is also added when there's a demonstrative noun instead of the definite article. See also here: https://www.duolingo.com/comment/3888221
As I understand it, it's the difference between "a gray egg," and "THIS gray egg."
When you speak of any random thing - "een wit paard," "een grijs ei," "een rode bal" - use the form that follows whether it is a het/de word.
When you speak of a SPECIFIC thing, "dit witte paard," "dit grijze ei," "deze rode bal" - always use the form that goes with a de word.
gray and grey are acceptable spellings, Gray is American English and this app is based on American English even though usually it does accept when you spell and write the British way. British spelling and sentence structure is what we study in Europe but not everywhere else in the world