Most of the CMS provides a color picker in admin panel to user to choose a color. And almost all of the color pickers return the color code as hexadecimal mode, e.g. #4EE4D3. So, you can just get the value and set the color in css. For example:
[php]
body{
background:
}
[/php]
This is a very good practice. Problem occurs if you want to put a opacity option to the user. You can set background opacity with hexadecimal color value. You must use RGBA filter, so RGB color mode as well. Like:
[css]
body{
background: rgba(233, 122, 234, 0.7);
}
[/css]
So, we need to convert the hexadecimal value to its equivalent rgb value. Here is a very small function that will do the job:
https://gist.github.com/bappi-d-great/d778d16e8220b5309582
So using this function you will get the rgb value, and with that you use the opacity 
Hope you will enjoy!