complementary color
If you stare at a bright red patch and then look at a blank wall, you see a ghostly green afterimage. Red and green are complementary colors: opposites on the color wheel, each the leftover when the other is removed from white light. In the chemistry of colored complexes this everyday idea becomes the crucial bridge between what an instrument measures and what your eye sees.
White light is a mixture of all visible wavelengths. When a complex absorbs one band of wavelengths — say it pulls out the green light around 500 nm in a d-d transition — the light that survives and reaches your eye is white minus green, which the eye reads as the complementary color, a purple-red. So the rule is simple: the color you observe is the complement of the color absorbed. A complex that absorbs orange looks blue; one that absorbs violet looks yellow-green; one that absorbs all of the visible band looks black, and one that absorbs none looks white or colorless. A rough color wheel pairs them: red with green, orange with blue, yellow with violet.
This is why a visible absorption spectrum and the observed color are two views of the same fact, and why chemists reason backward from color to electronic structure. See a green solution and you suspect it absorbs in the red, which (for a transition metal) tells you roughly where delta-o lies. The honest caveat: real complexes often absorb in more than one band, and the perceived color is the mixed leftover of everything that survives — so the single-complement rule is a reliable first guide, not an exact law, and the absorption spectrum is always the more trustworthy record.
[Cu(H2O)6]2+ absorbs in the orange-red region around 800 nm; the complement of orange-red is blue, which is why copper(II) solutions look pale blue. Add ammonia and the absorption shifts to shorter wavelengths, deepening the blue toward an intense royal blue.
Copper(II) absorbs orange-red, so we see its complement: blue.
Brightness and color are separate questions: the complementary rule tells you which hue you see, but how vivid it is depends on how strongly the band absorbs — and faint Laporte-forbidden d-d bands make for pale colors no matter what the hue.