Monochromatic means one color, and that's what this color scheme is - variations on sage green. Use darker versions, called shades, and lighter versions, called tints, of your sage green wall to create a calm and serene environment. Monochromatic color schemes are most successful when surface textures vary, so use a variety of fabrics on upholstered pieces.
Choosing three or four colors next to each other on the color wheel results in an analogous color scheme. One color should be dominant, with the other colors used in lesser amounts. Sage can be in the middle of an analogous scheme, in which case green and yellow would be the other colors, or it can anchor the palette. A cool analogous scheme would include sage, green, blue-green and possibly blue, while a warmer palette would have sage, yellow, yellow-orange and possibly orange. Rooms decorated in analogous color schemes naturally feel interesting and coordinated because the colors are all related to one another.
Complementary colors are those opposite each other on the color wheel, in this case yellow-green (sage) and red-violet (plum). Complementary colors create visual tension and rooms decorated with them tend to be very dramatic and energetic. The combination of sage green and dusty rose, a shade of red-violet, was a popular color palette in the 1980s.
Instead of using red-violet, sage's complementary color, choose the two colors on either side of it, violet (purple) and red, for a split-complementary color scheme. You'll still get the punch of contrasting colors, but without the visual drama caused by a true complementary color.
Three colors spaced equally apart from each other on the color wheel produce a triadic color combination. For this palette, you'd choose blue-violet and red-orange to go with your sage green wall. For example, if your walls are a medium sage green, you could use dusty blue and terra cotta as coordinating colors. Triadic color schemes are more complex; they're good choices for rooms with a lot of pattern in them. As with all color combinations, one color should predominate, with the other colors used in smaller amounts and spread throughout the room to unify the decor. Triadic rooms feel energetic because of the high contrast between colors.
For the most harmonious color schemes, combine colors of similar intensity and value. Intensity is how bright or saturated a color is. Sage green is a shade or grayed version of yellow-green, so all the other colors you choose to combine with it should also be muted and slightly grayed - that's why dusty pink combines so well with sage green, for example. Value is the relative darkness or lightness of a color, so if your sage green wall has the delicate lightness of new leaves, choose other colors that are soft and pastel. The exception to this would be for a monochromatic color scheme, where you are intentionally combining different values of the same color.