If you are talking about the color, you can say 红色 (hong2 se4). The word 色 means color, so saying this phrase refers to only the color.
- 我最喜欢红色 (wo2 zui4 xi3 huan1 hong2 se4): I like red the most
If you are talking about a red _________, using red as an adjective, you would simply say 红 (hong2), or 红的 (hong2 de1)
- 这朵花好红 (zhe4 duo3 hua1 hao3 hong2): This flower is so red
- 这是红的花 (zhe4 shi4 hong2 de1 hua1): This is a red flower
