If you are referring to the font color pallet that is available in the editor then you will have to edit the GenericControls.template.php (if the custom theme does not have that file, simply copy the one from the default theme to the custom theme)
In that file you will find
colors: "black,red,yellow,pink,green,orange,purple,blue,beige,brown,teal,navy,maroon,limegreen,white",
Change those to what you want, you can also use hex color codes ie #cccccc in place of the standard html color names.