"The color of this skirt is not good."
Translation:Die Farbe dieses Rocks ist nicht gut.
The "corrective text" (the bit that appears over my answer that shows what I did wrong) says I "used the singular 'Rock' here, instead of the plural 'Rocks'." But the plural of Rock is Röcke, and isn't relevant anyway because the English sentence I was asked to translate says "this skirt", not "these skirts".
EDIT: Ahh! Right! Rocks is singular . . . when in the Genitiv. The answer, dieses Rocks (or, I suppose, Rockes) is correct, but the corrective text was wrong. The correction should have stated "did not use the Genitiv, 'Rocks' or 'Rockes'".
Among other very useful bits, http://coerll.utexas.edu/gg/gr/cas_08.html states:
"masculine and neuter nouns receive an -s suffix at the end of the noun. BUT if the masculine or neuter noun is one syllable long, it usually gets an -es. "