A mini visual design guide about color and its use for user experience designers. There are a lot of resources on the internet about color but to get the whole picture you have to find a lot of URLs, wade through examples from print sources to find digital examples of color in action and hunt around for useful tools to help you determine color palettes, accessibility issues and tools for specifying colors for CSS.
This guide covers some early color wheel history, color theory and gives a bunch of examples of color in action across mobile and content sites and applications. There are more resources provided to dig in deeper.
One PDF file