Hi, I am a Native Chinese speaker. Your question is interesting.
In the example of 裙子是蓝色的。I do think 蓝色 is a noun, which means blue color.

But 蓝 alone can be adjective. You can simply say 裙子是蓝的。 So if you say 裙子是蓝的, it applies to the structure of Noun 是 adj. + 的。

But noun can modify noun in Chinese.
For example, 裙子是棉麻的。（The dress is made of cotton and linen. ）So here the structure is Noun 是 noun + 的。
So it also applies to 裙子是蓝色的.

Instead, to say “the dress is blue,” you would say, “裙子是蓝色的.” The formula is as follows: Noun + “是” + Adjective + “的.”

But is 蓝色 a noun in Chinese? and 蓝色的 all together is an Adjective? So, if 蓝色的 all together is an Adjective then you do not need another 的？

Also, for this part：

Another way you can use adjectives in Chinese is by applying them before nouns. In English, you might say, “The pretty girl.” In Chinese, that would be “漂亮的女孩.” (This uses de 的 to describe the person)

So, if we move the Adjective into the front, then the structure will be Adjective + 的 + Noun, but what about this 蓝色的裙子, again. 蓝色 is a Noun in Chinese, then 蓝色的裙子 sounds right. So my question is why 蓝色的裙子 can use this structure? Since 蓝色 is a Noun?