RGB vs. CMYK
RGB color is a color model in which Red (R), Green (G) and Blue (B) light sources are added together in different strengths and combinations to give a full array of colors. This color model works well for on screen intents because the same process is used to create color on the vast majority of screens.
In contrast, CMYK color is the primary color model for print. From home printers to professional publishing operations, printers generally combine cyan (C), magenta (M), yellow (Y) and black (K) to create dynamic color ranges on a variety of mediums.
Because these color modes are fundamentally different, it is always important to match the color model with the intent. This can be chosen when you create a document or by locating Image < Mode.
The Color Picker
In the bottom of the tool bar, you might have noticed two overlapping squares filled with black and white. While this tool hasn't held color until now, it is the primary place to select and edit colors. The two squares allow you to hold multiple colors at the same time. To use a color, simply click on the colored square that you intend to use. It is important to select the color before using a tool. If you have the wrong color on top, the tool will utilize the selected color.
For a more in-depth look at the color picker check out this video by Brooke Godfrey, UW - Madison
The swatches panel allows you to quickly select or save colors for use at another time. They can be grouped together for specific projects.
After getting a solid handle on picking and utilizing colors, it is a good idea to get comfortable with gradients. Gradients are a great tool to start to mimicking real-life textures (ie. metals) and develop depth in a project.
Source: PeachPit.com via Adobe.tv
Tip: Make sure you remove and float the swatches panel from the panel dock when working with gradients. The easiest way to add specific colors to the gradient slider is to drag them from the swatches panel.