The inspiration for Hiro came from a simple black to white gradient. The easiest is sometimes the most beautiful, especially if you are rather partial to one color.
I like the idea of choosing colors based on what makes your eye happy -- just winging it -- but not everyone is confident of their choices. The Color Collective blog is a brilliant resource for teasing out what makes a combination work. You could browse her posts for hours, admiring her dissection of all the pretty pictures, but mostly, you can look at the colors and get excited about putting them together in a sweater.
I love these colors together. They remind me of vintage Baskin-Robbins signage, and I loved Baskin-Robbins as a kid (their brand identity materials are gawd awful these days, I won't link to them to save you from the eye searing uggly of it all). But I might not be able to get them in a single yarn line. No worries, I have other ideas too.
There are many places in books and on the web that offer insight into choosing colors that go together in a pleasing way. It all boils down to relationships, how colors relate to each other on the spectrum. Even though the eye knows best, there are almost always clinical explanations for why you find something pretty. In the case of color, the shorthand for this explanation can be found in the color wheel (borrowed from here):
Without getting into the knitty gritty of color theory (let's just say that there are lots of people with much more road under their tires on this subject that there is under mine), I'll offer a few tips here, and you can explore on your own, and certainly ask me in an email if you need advice or reassurance. You can also refresh the basics with a good google search on color theory, or you can start someplace like this.
It's a good way to start, if you feel helpless about choosing colors yourself, to settle on your favorite color and use it as a guide for finding other colors, based on their relationship to your favorite on the color wheel. Say you love purple.
Neighboring colors, also know as analogous colors, are a nice direction as you can see on the left. These work well because they are close to each other on the color wheel and related to each other because of their proximity. And on the right I have a set that is based on the primary colors of red, blue and yellow. In this case, I have purple and orange, both secondaries of red, and yellow, which is the complement of purple, making a nice visual balance as far as the wheel goes.
I could get into split complementaries and the like here, but you get the idea. I could look for balance or proximity to explain why I like something, but I do consult the wheel if I have two colors I love, and can't settle on a third or a fourth choice. I have tended to choose two pairs in my color selections so far, each pair analogous with each other, and either analogous or complementary of the other pair. Two different purples (wine purple and hot violet pink) with two different reds (red and red orange) in the original Hiro make it a wholly analogous color family choice.
The colors I chose for Hiro Petite are more complementary, spread even out around the wheel from each other: yellow, pink, and green:
The other thing that feels important to me about colors for this sweater is that they should all have about the same intensity, or tone as they say in color theory. Which isn't to say that a pastel blue wouldn't be beautiful next to a chocolate brown, but it would probably be prettier to me were the blue a little more intense, like a turquoise or a Carribean blue. But that may be just me.
Remember that I am just offering this as a clinical explanation, and you may not find this works in real life when you're standing in the yarn store or in front of your stash piled on the guest bed. Above all, trust what you like, swatch if you feel insecure, and email or Rav message me if you have any question. Meanwhile, I have posted on Ravelry a bunch of swatches for both sweaters that I made using the excel skills I learned froml Marnie's excellent tutorial. Feel free to consider them as a starting point in your own exploration.
I'll leave you with this: I happen to like these combinations for a more earthy take on this wild color party, and I can't find most of these colors on your garden variety color wheel. How crazy is that? Not really at all. Browns and beiges are really just deeper saturations of pure colors. The dark browns here are just very deep greens, and the beiges are either yellow (on the left) or green (on the right) in their origins. If you look at these studies in that way, you can understand that they work because the one of the left is a split complementary of yellow, red, and green, and the one on the right is just a bunch of greens together. For more about how this sort of thing works, I suggest you consult a bigger resource like Johannes Itten's The Art of Color, or Color and Fiber by Patricia Lamber, Barbara Staepelaere, and Mary G. Fry.