I thought you would download the picture, grab the color, then use it on the site (if they let you input a code).
Since I'm not the fancy type, I've only used black and white for text. Let me go check if they let you specify a code...
*checks*
They do. Click on the white color box and you'll see a code: #ffffff. This is the code for white in hexadecimal. Click on the black color box and you'll see the code #000000 for black.
So you can grab colors on your laptop, then specify them in that box. Make sure to use the prefix '#'. The code represents how much of red, green, and blue the color has.
All zeros, means there are none of these colors. All 'f' means each is at the max. 'F' is the highest digit in hex. All max value creates white when mixed together.
More specifically, the six digits are broken down into this layout: RR GG BB. The six digits are mapped onto that template and blended by the software when creating a color.
Many websites have elements specified with this scheme and the color is rendered on the fly, each time the page is displayed.
Oop, almost forgot. The codes are not case sensitive. #ffffff is the same as #FFFFFF.
And there you go. That's enough info to match your colors exactly. There are hex color charts on the web if you have a color name (eggshell, firebrick red, hunter green, etc...) and want the hex code. Check it out if you're interested.
And have fun with colors. :-)