First install with sudo aptitude install gpick, and start it with gpick;
Build a color theme based on existing web page
Choose "Color picker" tag.
Select a slot in color picker swatch (the six hexagons mark by 1 ~ 6). Start color picker by shortcut Ctrl-p, move mouse to the location, press SPACE to put the color into the slot.
The highlighted slot become the next slot automatically.
So you can select another color, and put it into the current highlighted slot by press SPACE.
Right click the color picker swatch and select "Add to palette" or "Add all to palette" to put these color into the color palette on the right-hand side.
Generate Color theme based on existing color
Right click one or more colors in the color palette, select "Generate...". Set "Colors" to 3, "Type" to "triadic", you can find the triadic color group in "Preview" panel. If this is what you want, press "OK" button.
In the same way you can build complementary color group with: Type: Complementary, Colors: 2.
Build a color theme from scratch
Choose "Scheme Generation" tag in upper-left corner. After you've decided the value of hue, saturation, lightness and type, right click the color block and select "Add to palette" or "Add all to palette".
Find all built-in shortcuts on the "HELP" section of gpick's website.
Ref: Creating color schemes with Gpick;