The Color Window is available at the 'Windows' pull down menu. The hotkey F12 of the default key map opens and closes the window. The color gadget of the color window is used as a component in many property sheets, tool controls etc. in a reduced form (without 'Predefined Colors' array). The documentation below is valid for all such color gadgets.
The floating color window opened from the 'Windows' pull down menu does not directly change any color attribute in the scene. It acts as a tool where you can blend colors and optionally store them to the 'Predefined Colors' array. The actual use of created colors happens by dragging and dropping colors from the color window to color gadgets of object property windows, to tool controls etc. Colors can be fetched into the color window the same way, using drag and drop.
The following controls are available:
Color Wheel: You can pick a suitable base color from the circular color wheel gadget. The high, narrow color sample rectangle immediately at the right side of the wheel displays the result color while you LMB drag the mouse on the wheel. The slider at the right adjusts the darkness of the selected color. The color wheel acts as a drop target: dropping a color from another color gadget onto it sets the current color.
Current Color Area: This rectangular area displays the result color defined using other controls of the window. Despite of its small size, it provides some useful functionality:
A LMB click opens the standard color dialog of the operating system. You can use it as an alternative way to define the color.
A single popup menu 'Expand' is available on it. If cleared, the color wheel and related controls collapse into a minimal state where only a single numeric rgb(a) field and the current color area visible.
You can drop colors into this area and drag the color from it into other color gadgets.
Brightness Bar: The vertical bar on the right adjusts the brightness of the selected color. The scale starts from black and ends to the color specified using the wheel. LMB click or drag the bar to change the brightness.
Red, Green, Blue, Alpha: The numeric components of the current color. In the expanded state, the values can be controlled using separate sliders. In collapsed state, a single field defines values, sub components being separated with empty spaces. The alpha channel is optional and not available in all contexts. Some tools, which need the alpha value, show the gadget with the alpha control. For example, the 3D paint airbrush uses the alpha value to define paint transparency.