If you have never used Agave before, you'll need to install it:
Now that you have Agave installed, go ahead and open it up by selecting the Applications → Graphics → Agave desktop menu item.
At the right, you'll see a screenshot of Agave. Your Agave window should look something like this, although you likely won't have any favorite palettes saved in the right panel.
The first thing you'll want to do is to choose a base color for your palette. From this base color, Agave can automatically generate a number of different harmonious color palettes using multiple techniques. So let's pick that base color!
There are two main ways to pick a base color, using the color chooser button or using the palettes along the bottom of the window.
Click on the small button on the left side of the Agave window, immediately above the 'Palette' label. That will bring up the Pick a Color dialog.
You may drag the small black circle within the triangle on the left of the Pick a color dialog to change the color's shade, and drag it circularly around the rainbow to change the hue of the color.
Keep the Pick a color dialog open. Open up the graphic you'd like to pick a color from. It can be in a browser window, image viewer, or email client - it really doesn't matter, it just needs to be on screen. Now, click on the small eyedropper icon button in the lower left of the Pick a color dialog. Your mouse pointer will turn into a little color dropper icon. Hover your new dropper mouse pointer over the graphic you want to pick a color from, and click on the area that has the color. The Pick a color dialog will update and show the color you picked.
Using the Color name: field in the Pick a color dialog, you can enter in HTML Hex color codes (They look like this: #FFCC00). You may also enter in manual Hue/Saturation/Value or Red/Green/Blue color values. This is helpful if you've picked a color in another graphics program or in HTML and want to generate a palette of more colors that will work with it.
Once you've picked the color you like, click on the OK button in the lower right of the Pick a color dialog to set the color.
Agave comes pre-loaded with a set of large color palettes. They include "Visibone," "Tango Icon Theme Palette," "GNOME Icon Palette," and "Web Safe Colors." There is a Palette dropdown in the lower left of the main Agave application window. You can pick between these palettes from that dropdown. Once the palette is visible, you can hover over the colors in the palette to view their HTML hex codes, and you can click on any color with the hand icon to set that color.
Now it's time to dig into your base color and discover what kinds of color palettes you can create with it. There are three main ways you can work with your base color to generate palettes - changing the color scheme of the palette, changing the saturation of the palette, and changing the brightness of the palette.
You can change the number of colors in the palette associated with your base color by changing the scheme between the colors. Depending on the relationship you choose, Agave will select between one to three companion colors for your base color. Here's a summary of the color schemes you can choose from. Choose between these via the dropdown in the center left of Agave.
Once you've picked a color scheme you like, you can tweak the saturation of the palette by pressing the up and down saturation icons in Agave as shown in the screenshot. Toning down saturation in a color palette can convert your palette from having very deep rich colors, to having very delicate and slight tones. Changing the saturation of a palette is a technique you can use to change its mood.
Tweaking the brightness of a palette is another way to change the mood and feeling of a color scheme. By pressing the up and down brightness icons in Agave as shown in the screenshot, you can turn the brightness of all colors in the palette up or down.
Once you've got a palette you're happy with, the simplest way to save your palette is to press "PrtSc" on your keyboard to create a screenshot of the color scheme that you can print and bring to the store to pick out clothing or decorations for your home.
However, it's very useful to keep a list of your favorite colors handy in Agave. You'll notice a screenshot of the Agave favorites pane. To save your full palette here, color-by-color:
You can save all of your favorite colors in the Gimp palette file format (GPL) by visiting the File → Export Favorites... item in the Agave main menu. You may import this palette into the Gimp graphics application - the Gimp manual has more information on this process.
Now you know a little bit about how to create harmonious color palettes by exploring Agave. Have fun, and be sure to share these tips you learned with others!