The Color Picker window is displayed when you need to set a color value in
MainActor Sequencer. There are several ways of picking the color. You can click on
the color in the left side of the window (to set the hue and saturation) and
then on the black to white column of luminosity values. You can also set the
desired RGB/HSL/YUV values using the spin buttons in the tabbed panels on the
right. Finally, you can click on one of the user-defined colors at the bottom right.
To change the user-define colors, select a new color using one of the above
methods. Left click on the large Color button and drag it to the small
user-defined color button that you want to change. You can also drag the user-defined
color buttons to the current color button or to other user-defined color buttons.
(The latter is useful for rearranging the user-defined color buttons.)