I want to customize the red color in change the look, it's not exactly the color I want in try it out. How should I do this?
You can also use the SharePoint Color Palette Tool. It helps to build your custom colors or just make a copy and adjust one of SharePoint own palettes.
You can do so by making your own composed look (You could alter an existing composed look). You can go to Site Settings, and under Web Designer Galleries, click composed looks. There you can find the definition for the color for each font under the column: Theme Url. You can download this file, it's an xml file so you can open it in Notepad, and adjust what you like, then you can upload it to the same location which is (/_catalogs/theme/15/). Make sure you change the name so you don't override an existing file.