Choosing colors that work together harmoniously isn't always as straightforward as choosing hues that should work well together. We've been looking at how certain combinations such as analogous or complementary palettes are almost always a success - and that is definitely more than half the battle - but if you follow up that thinking by spending a little time considering the undertones of the exact versions of the color possibilities you are choosing from to make up those palettes, I guarantee that your room will look even more polished, more beautiful and more pulled together. Here's what I'm talking about...
Every color has an undertone and it's important for the versions of the colors you choose to have harmonious undertones, as well as that good color combination relationship (such as analogous or complementary, etc).
Here's one way to think about it: no color exists in isolation. That is why undertones matter. You can love a specific gray on a paint or upholstery fabric swatch, but what matters is how it works with the other colors that are part of your design. When its isolated out in a swatch, a shade of let's say, gray, is what it is and it either appeals to you, or not. You might not even be able to determine what the undertone of that shade of gray even is when you look at it on its own. But, once you put it next to other, similar grays, the undertones will start to become clearer.
Let's say you have 8 possible dark gray paint colors to choose from. They are similar enough that they all can be described in exactly the same way ("I'm painting my bedroom a dark blue-ish gray"). But, put all eight of those swatches next to each other, and you might see that some of the blue-grays lean a little toward purple, others more toward green.
Once you start to determine the undertones and can describe them as they appear, such as "a green-ish blue gray " (one with more yellow undertones) or a "purple-ish blue gray" (one with more red undertones) and you'll have a clue as to which one will likely work best with your other color choices for the space.
For example, that more purple-ish gray might be a more harmonious choice in a palette that includes pinks or reds (or other colors/neutrals with undertones of those colors), whereas the slightly greenish blue gray might work better in a color scheme along with more greens and yellows (or neutrals with undertones of those colors). Alternatively - but still successfully, depending on the effect you are going for - sometimes you may want to go for the opposite undertone, such as in a room where you are going for a true complementary color scheme, where you want to play up the opposite-ness of the colors as much as possible, including their undertones.
Now, this can sound mysterious and complex, and it IS somewhat designer "insider baseball" to talk about undertones, but in reality, the takeaway - the thing that should increase your color confidence - is that you know the importance of not committing to that seemingly-perfect "favorite" blue-gray that you love on a swatch, in a vacuum. Pick up multiple swatches, compare them to each other AND, most importantly, to the other colors in the color scheme you are working toward and then determine which one is truly your best bet.