An accent color is the color you'd call out if asked "what color is this room?" — even when it occupies only 5-10% of the visual area. The dominant neutrals (whites, beiges, greys, woods) provide the canvas; the accent color provides the personality.
Good accent colors share a property: they're saturated enough to register against the neutrals without overwhelming them. Dusty rose, deep teal, mustard, terracotta, ochre, forest green, navy. Pure primary colors (fire-engine red, electric blue) work occasionally but read as more contemporary and harder to live with long-term.
The placement: accent color appears in 3-5 spots minimum, scattered around the room rather than concentrated. A single rust-colored pillow looks accidental; a rust-colored pillow plus a rust-toned art piece plus a rust ceramic on the shelf reads as intentional.