What exactly and better will be is to submit the dropdown and check which color is submitted. Based on that particular color chose your css file.
e.g. you create 3 css files named as blue-bg.css, green-bg.css and black-bg.css
then just put the related colors in those css files. e.g.
body {
background-color: blue; // same code but change the color for all files
}
Then in your header file you can check the posted value of the dropdown and load the css file based on the condition you submitted.
Hope it helps you out.
Regards