PHP is a Hypertext Preprocessor. This essentially means that PHP dynamically writes web pages for you. So it can query a database and print a report from it, or change CSS and such based on specific criteria.
You can do some interesting things with color on your HTML pages with color options with CSS. Color Picker: https://www.w3schools.com/colors/colors_picker.asp Color Names: https://www.w3schools.com/colors/colors_names.asp color.html colorStyle.css