February - last edited February
@Snakitten When your Alien Sim disguises him/herself the other colours will come available in the 'extra' Sim. Then you can also use the tone slider to make a colour you see fit for your Sim. Alien Sims are all created with the colour palet in your second screenshot (142). When and if you want extra colours for your alien Sim you should check out some mod pages.
Have fun playing. 😉