Because it is the color of the bear (specific), you use dell', but if it was just bear colored in general (ha), it would be d'
Does that make sense?
The audio says "Il colore dell'orso" but the translations says "È il colore dell'orso". You should change it accordingly.
This time Duolinguo accepted the UK English spelling of colour, whereas previously English spellings have not been allowed. Whenever there is a difference between Am Eng and Br Eng, both should be allowed. Both are surely valid?