The Colors Palette lets you pick, sample, and store colors in a variety of ways. To open the Colors Palette click on the front or back color well, choose the Window ▸ Colors menu item, or use keyboard shortcut ⇧⌘C.

Selecting Colors

Along the top of the window is a toolbar with various options to pick colors. There are wheels, sliders, spectrums, crayons, and hexidecimal color codes. Playing around with the sliders is one way to get results.

If you already have a picture with the color you want then click on the magnifying glass, put the crosshairs over the color you want, and click once. This will set the color to the one you just sampled.  An alternative way to bring up the magnifying glass is View ▸ Select Color.

When you have picked a color, it will show up in the color bar next to the magnifying glass. 

Sometimes there's a mismatch between the colorspace that the color picker is using, and the colorspace that the image is using.  This results in a color showing up that doesn't match the color that was picked.  You can find the color panel's colorspace by clicking on the color space button next to the RGB sliders.

And for the image, you can find out the color profile by selecting the Image ▸ Assign Color Profile menu item.  Set the color profile so that the color picker and image color profile are the same.  

An additional method to try if if the magnifying glass is getting a color mismatch, is to select one of the drawing tools such as the brush, pencil, or fill tools.  Hold down the option key to bring up the color picker, and select your color in the canvas. 

Adjusting the Opacity

You can adjust how opaque / transparent your color is by adjusting the opacity slider at the bottom of the color palette. 

A transparent color will have a vertical slash through it in the color bar.  The top portion is what the original color looks like without opacity, and the bottom is with the opacity applied:

Pro Tip:  If you want to see both the color and opacity level of an existing color in your image, select the brush tool then hold down the option key while clicking on the color.  The color and its opacity will be displayed in the colors palette.  

Swatches

Colors can also be stored by dragging the color from the color bar down onto the grid at the bottom of the palette.  If you don't see the grid, pull on the bottom part of the color window as if it was a drawer. 

Keeping a swatch library can save you time later if you consistently use the same colors.  Any color swatch that you drag down into this section can be accessed from most other applications on your computer.  If you have a green you use on all of your stationary, you can store it and use it in VoodooPad, Keynote, Pages, Mail, iWeb, etc.