I have the BIG bottles of white, black, green, and the primaries (red, blue, yellow). With that, of course, you could theoretically create any color you want! But for convenience I also have, in the regular little bottles, a couple of different shades of orange, "egg yellow", violet, "regal purple", several different shades of green, a couple of blues, "chocolate brown", ivory, and "deep pink." Um, and mauve, and possibly some other things I'm forgetting.
Ivory and pink together make a good (fair) skin color, and I often use different shades of green and brown for grass/leaves/trees. Blue and green are favorite colors in my family, so it's good to have a variety of those. Pinks and purples tend to come in handy for girly birthday cakes, and for some reason orange tends to come up a lot in my designs, for both boy and girl birthday themes and for more adult-oriented designs.
Anyway, if you are wanting to get colors that are different from the Wilton you already have, I'd suggest trying any or all of the "electric" colors, orange, forest green, fuchsia or deep pink, turquoise, navy blue, ivory or peach or mauve. Maybe terracotta (reddish orange) or one of the other red shades.