I'm not sure what aspect of programming you're asking about, but when people compose HTML/CSS code for websites, the term "hex code" usually refers to the way colors are encoded using 6-digit hexadecimal (base-16) numbers. For example, the hex code 8B9A18 (or equivalently 8b9a18) corresponds to the green color you see on "Follow" and "submit answer" buttons on Hubpages.
A nice site that shows a full palette of colors along with their hex codes is here:
If you want to select you own colors using a color wheel and see the corresponding hex code, you can use this tool here:
When you're specifying the color in an external style sheet or using in-line CSS, you write it like this, color:#8B9A18;
Hope that answers your question.