Use the 60-30-10 rule
60% dominant color (usually walls and floor), 30% secondary color (sofa, curtains, rug) and 10% accent (pillows, art, vases). It's a simple guide to make the living room look balanced, not cluttered.
Think about what you already have
The sofa, floor and large furniture are the hardest to change. Build your palette around them: if you have a gray sofa, a sage or soft terracotta wall works better than another gray. Bring a photo or fabric sample of the sofa to the paint store.
If the living room is open to the dining room or kitchen
In open-plan spaces, use the same wall base throughout and define each area with furniture, rugs or an accent wall. That way the whole space looks cohesive instead of like three rooms stuck together.