These operators let rules and actions manipulate Color-type attribute values. It can often convenient to tie the colour of a note to its properties. For example, you can make overdue notes red, with a brightness proportional to how overdue they are.
Note that when setting a shade of a named colour in a string the shade value (lighter, darkest, etc.) comes before the colour, unlike listings in Get Info's attributes tab and the Inspector (e.g. Appearance Inspector, Interior sub-tab); thus "dark warm gray" not "warm gray dark", "light cyan" not "cyan light".
$Color = brightness(hue(RGB(255,0,0),120),75)