黑色 (hēisè) is a noun meaning "black" or "the color black." It is used to describe the color of objects and also appears in figurative expressions such as 黑色的一天 (a black day), meaning a dark or tragic day.
她今天穿了一件黑色的大衣,看起来很帅气。
Tā jīntiān chuān le yī jiàn hēisè de dàyī, kàn qǐlái hěn shuàiqì.
She wore a black coat today and looked very sharp.
那辆黑色的车是谁的?
Nà liàng hēisè de chē shì shéi de?
Whose black car is that?
那一天对公司来说是黑色的一天,损失惨重。
Nà yī tiān duì gōngsī lái shuō shì hēisè de yī tiān, sǔnshī cǎnzhòng.
That day was a black day for the company, with heavy losses.