Shash łizhin means 'bear is black'. Black bear should be shash łizhinii or łizhinígíí.
Yáʼátʼééh! So, I think the English sentence needs to be fixed here. Thanks a lot for clarifying this. Please, remember always to send the report. :)
How come "Dibe ligai" means "the sheep is white" (and when I put "white sheep" it was incorrect) but Shash łizhin is "black bear" and not "the bear is black." ??
Yes, this is an error of Duolingo.
Łizhin and other color words are verbs, meaning "to be + color". So Shash łizhin means "bear is black".
To say "the black bear" in Navajo, one needs to turn the verb to a relative: "the bear that is black". To do so, one can add -í, -ii, or -ígíí to the end of the verb: Shash łizhinígíí.
That is also how "orange" the fruit is constructed: ch'il łitsxooí, is actually "the plant that is orange", with łitsxo meaning "to be orange" and -í the relativizer.