Complementary colors sit directly opposite each other on the color wheel (red/green, blue/orange, yellow/purple). Placed side by side at full saturation, they create maximum visual contrast and seem to “vibrate” against each other.
It's how artists like Van Gogh and the Fauvists made color itself feel emotionally charged rather than just descriptive of local color.
Choose the main color your piece will be built around.
Locate the color directly opposite it on a color wheel.
Use roughly 80% of your dominant hue and 20% of its complement, rather than a 50/50 split, so one color still reads as dominant.
Use small, sharp touches of the complementary color exactly where two shapes meet, not spread evenly across the piece.
Avoid graying down your focal area by mixing the complements together there; save any muted mixing for background areas.
Step back and squint at your piece — the complementary passages should visually pop or vibrate against their surroundings.
Paint the same simple still life twice — once in any colors you like, once forcing yourself to use only one hue and its direct complement — and compare how differently they feel.