The colour wheel is just a map of how colours relate to each other, and once you understand three relationships on it you can stop guessing.
Complementary colours sit directly opposite — blue and orange, red and green, yellow and purple. Pair them and they make each other sing, which is why a terracotta cushion looks so alive against a soft blue wall. Use this for accents and contrast, not whole walls, or you'll fry your eyes.
Analogous colours sit next to each other — say green, blue-green and blue. These give you calm, restful schemes because there's no fight between them. A room moving through Farrow & Ball Treron into Inchyra Blue is doing exactly this, and it feels effortless because the wheel's already done the maths.
Monochromatic schemes use one hue at different strengths and tones. Little Greene's colourcards are brilliant for this — they often print a colour in pale, mid and full strength so you can build depth without introducing a single new hue. Setting Plaster on the walls with a deeper rose on the woodwork is the same idea.
The "but what about neutrals?" question always comes up. Neutrals aren't really off the wheel — they lean. Cornforth White has a cool green-grey undertone; Skimming Stone leans warm. Knowing which way a neutral leans tells you what it'll do next to a colour. A grey with a green undertone next to a green wall reads harmonious; the same grey next to a pink can look dirty.
Practical advice: don't choose a scheme purely on paper. The wheel tells you the *relationship*; your north or south light tells you how it'll actually behave. Pick your relationship first — calm analogous or punchy complementary — then sample big boards and live with them for a few days through the day's light. The wheel narrows the field from thousands to a handful. The wall does the rest.