你穿红色的很好看

The order is 因为 + reason, 所以 + result (yīnwèi + reason, suǒyǐ + result). The first clause explains why; the second tells what follows. In 因为你没穿过红色的，所以要试一试 (yīnwèi nǐ méi chuān guo hóngsè de, suǒyǐ yào shì yi shì), lack of experience is the reason and trying the red pair is the conclusion. In a direct answer to 为什么 (wèishénme), a short 因为 (yīnwèi) clause may stand alone because the result is already understood.

You already know how a phrase can describe a noun: phrase + 的 (de) + noun, as in 新开的商场 (xīn kāi de shāngchǎng), “a mall that is newly opened.” In this lesson, phrase + 的 (de) can stand on its own as a noun phrase: 新开的 (xīn kāi de), “the newly opened one.” The complete phrase ending in 的 (de) refers to a person or thing identified by the description. Context tells the listener what it refers to; it may be something already mentioned or something obvious in the situation. 的 (de) does not itself mean ‘one,’ ‘what,’ or ‘who.’ Those English words translate the meaning of the whole phrase in context.