One of the more difficult decisions I've seen customers have to make is when they are selecting two or more colors for a project and can't quite decide what would and would not go together.
While a basic understanding of color theory can help in this department, (for more info, click here), if you are having a bit of difficulty here are a couple of tips to help you figure out coordinating colors.
A relatively easy way to begin thinking about coordinating colors is to take your cues from variegated or speckled yarns. Dyers and yarn companies created these colorways because the colors go together in some way. At times the colors might contrast, so they'll make one another stand out a bit more. Othertimes, the colors are more coordinating, so they mix and blend in ways that are soothing. What's helpful about examining speckled or variegated yarn and fiber is that you can get a sense of which colors work well together for the effect that you are aiming for. If a pattern calls for contrasting colors, then check out yarns where the multiple colors really stand out distinctly from one another. This means that when they are used together, those colors won't become "muddy" or difficult to see.
If, though, you are choosing both a solid and a variegated, generally speaking you want colors that will contrast with one another, not coordinate, which means you'll want to avoid selecting a solid that matches the colors in the variegated or speckled yarn you are using. For example, if you have a highly variegated yellow/red/orange/brown yarn that mimics fall leaf colors, you will want a color that offers you high contrast so that the variegated colorway doesn't become lost, such as a black, charcoal grey, or white.
Or, let's say you select a pink/purple/cream/teal yarn. Again, a dark solid such as black or grey might work, or you could select a royal blue that wouldn't obscure the teal but bring out the pinks and purples.
When looking to pair solids, though, you might take cues from a variegated yarn you like. If it pairs greens with creams and a little medium grey, then you know similarly colored solids would also look nice together.
Another trick is to hold up yarn colors next to one another to see if they bring out the hues and shades that you like, or if they don't look nice next to one another at all.