Adding an Easy Way to Customize a WordPress Colorscheme With Custom Post Meta

In my recent post, On Post-specific Styling, I used a plugin called Art Direction to achieve a post-specific colorscheme, but now I made some changes to my theme so that I can just enter a color into a custom field. I still have the plugin so that I can make more complex changes, but it is much faster and easier to enter colors. This post uses my method.