Pick the dominant color from the couch to use for the window treatments. For example, if the sofa has a medley of soft blues, subdued lilacs and a powerful pop of fuchsia, choose a fabric in the vivid powerhouse of a pink to create curtains for the room. Using a dominant coordinating solid for a large feature, such as the window coverings, ties the busier piece in visually. Use this color, as well as two other hues from the couch, in smaller splashes around the room with ornaments, minimalistic artwork (which can have a simple flower motif) and a pillow or two for further harmony.
Select the two main colors from the floral rug and use a quieter version of these for a couch throw-blanket, additional pillows, coffee table runner and chair or loveseat. As a reference, if your rug has a strong purple and blue theme, find fabrics with sparse, wispy prints of lavender and sapphire over a mostly white palette. This downplayed effect lends a hint of continuity without causing decorating dysfunction.
Search both the rug and couch for a similar background hue. Likely, this will be a neutral or subtle hint of color. Find a matching paint tone and use the cohesive shade on the walls. This pairing effect brings the two dominant floral pieces into airy accord.
Add rich dark wood tables and accents to a room with a pale color palette, or crisp white furnishings to a space saturated in color, to complete a balanced feel.