Those are defined in the GenericControls.template.php in your theme directory (or the default theme directory if you do not have a custom one). In that file (line 80 for me) you will see
colors: "black,red,yellow,pink,green,orange,purple,blue,beige,brown,teal,navy,maroon,limegreen,white",
If I recall, you can change/remove those to whatever you want as long as they are HTML defined color names. So you should be able to use any of the CSS1/2/3 names as seen here: https://developer.mozilla.org/en-US/docs/Web/CSS/color_value