红色(hóng sè) - red; the colour red in Mandarin

红色 (hóng sè) is the noun "the colour red". The bare character 红 already means red on its own, so 红色 is what you reach for when you are naming the colour rather than describing a thing: 我最喜欢的颜色是红色 (my favourite colour is red).

In front of a noun, both work, but 红色 wants 的 after it: 红色的车 or 红车 (the shorter form is more common in fixed compounds like 红酒, red wine). If you are unsure, 红色的 is always safe.

Worth knowing culturally: red is the colour of luck, weddings and new year money packets in China, not of danger or anger. 红人 (hóng rén) is a favourite or a rising star, and 走红 means to become popular, both from that same positive sense.