Differences are what make art interesting and engaging. There are infinite possibilities for differences. Differences of scale, texture, value, and color are just a few. If we want to showcase something in our art, it’s helpful to place something different, or opposite, nearby.
For example, if we have a shape that we want to feel very big, we can place next to it a very small shape. This will make the big shape feel even bigger. This small shape can be very small, however, if it is too small, say the size of the period at the end of this sentence, it will be SO different and unrelated, that the viewer will no longer make a comparison, and the potency of this difference will not be noticed. The period-sized shape, and the very large shape, are simply too different. They become unrelated.
Differences of color can work this way too. If the colors in your art are mixed without any of the other colors in them, they may feel unrelated. Colors that are even slightly harmonized will feel more related, and make your art more cohesive. It is completely fine to use unrelated or harmonized color in your art. It is only important that you understand both.