Indigo and White
Indigo is one of the oldest dyestuffs still in common decorative use. Before synthetic equivalents arrived in the late nineteenth century it was extracted from the leaves of plants in the Indigofera and Isatis families, fermented into a vat, and worked into cloth by repeated dipping. Each dip deepens the colour, which is why old indigo textiles so often show a range of blues within a single piece rather than one flat tone.
Paired with an undyed white the colour behaves unusually. Most strong hues shrink a room; indigo against a plain ground tends instead to sharpen it, because the eye reads the two as figure and field rather than as two competing colours. That is the logic behind the blue-and-white rooms of coastal houses, where the pairing does the work of a pattern without needing one.
In practice the difficulty is choosing the white. A blue-white or optical white will make indigo look cold and slightly synthetic. A white with a little warmth in it, closer to the colour of unbleached cotton, keeps the blue looking dyed rather than printed. Test the two together in the light the room actually gets, and look at them in the evening as well as at midday, since indigo darkens quickly as the light goes.
The pairing is also forgiving of mixture. Indigo sits comfortably beside faded reds, natural rush, unpolished brass and almost any wood, which is why it survives so long in rooms that are assembled over years rather than decorated at once.