Translation:She likes that blue dress.
Dress is - 连衣裙 liányīqún, Skirt is: 裙子 - qúnzi, why are they translating it as dress?
In adjective + noun structures, 的 is usually omitted if the adjective consists of only one syllable (source). Thus it's either 藍色的裙子 / 蓝色的裙子 or 藍裙子 / 蓝裙子.
Regarding 色, it seems that 色 can be omitted in adjective + noun structures, but not in noun + 是 + adjective or noun + 是 + noun structures. Thus it's only 這條裙子是藍色的 / 这条裙子是蓝色的 (lit.: this skirt is of blue colour) and 我最喜歡的顏色是藍色 / 我最喜欢的颜色是蓝色 (lit.: the colour that i like the most is the colour blue). Besides if an adjective and a noun form a term, like e.g. 綠茶 / 绿茶 (green tea), 色的 isn't used. (Unfortunately i couldn't find a reliable source for this, so take this with some caution.)
Typically single syllable adjectives don't use a 的 with a noun. However, there are instances when a noun has both a 1 syllable adjective AND a measure word that the 的 is used. There are exercises on Duo that use this format.
A native speaker would probably know the reason why.
I'm not a native speaker but one of the best Mandarin Chinese dictionaries on the net says it means a skirt or a dress. Check it out here:
Since you are doing a number of languages, have you also noticed the wide variation in the amount of questions there are in different language courses?
I'm also doing French and Italian, but there with each drill, there is only 7 questions and you get 10 points, and an extra 5 is added if you get all the questions right. Here with Chinese you have to do 20 questions on each drill to get he same marks. It takes quite a few hours to get 100 points in a day on Chinese, but with French or Italian it takes only a short time 15-20 minutes to get 200 points.
That is not a very balanced or fair approach.