In a recent Landmark Special interest Group the people there wanted to know how to create custom colors. If you have photoshop you can use the eye dropper tool to pick up the color. If you do not have photoshop, you can use a website to do the same thing. I often specify colors from a company called Resene. They have a Vectorworks color palette that you can install and they have color utilities on their website.
One of the utilities is a color picker. You can specify the URL of a photo online, or you can upload your own photo. Then you click on the image and the website will give you the RGB values. These values can be used in Vectorworks to create the color.
Here is a manual on creating custom colors in Vectorworks.