If you're talking about which order to put them into your palette, I put mine in "rainbow order". That is, starting with like, reds and progressing through the rainbow until I get to purples or neutral colours. It's really up to you though, another thing you could do is put all the most commonly used colours on one side. The palettes themselves get a little dusty from using the shadows, so putting the most used ones all on one side will minimize the cleaning you will have to do (if you're a big clean freak like me).
As for additional colours, I think you definitely need a black. Most people freak out at the idea of a black eyeshadow, but it's not like you have to wear it all over your lids. Black is more useful than people think; you can use it to add depth to other colours (for example, you can apply a tiny bit of black to the outer V of your eye with a blending brush, then put another colour over top so that the shadow will appear darker), and you can also use it to set eyeliner. I like wearing black eyeliner on my waterline, but I have a hard time keeping it there if I don't apply the liner, then set it with a black shadow. The two most popular MAC blacks are Carbon and Black Tied. Carbon is a matte shadow so it is a bit harder to work with than Black Tied, which has pretty sparkles in it. I personally prefer Black Tied for two reasons; I like doing the black eye thing and I also like for my black to be sparkly (obvious reason), and the sparkly formula makes it easier to blend when using it in my outer V, and once another colour is on top, you really can't see the sparkles anymore anyway.
As for an actual COLOUR, haha, I think something purple or green would be a good addition to what you already have. Reddish colours are probably best on you, but you already have a few of those. Humid is a good green if you want to do a smoky green sort of thing (I'm partial to smoky eyes, but I'm not really sure if you want darker or lighter colours), swimming and juxt are good for lighter greens. As for purples, Parfait Amour and Purple Haze are my personal faves.
Good luck!! HTH
