A happy accident of math: 4,096 × 4,096 is exactly 16,777,216 pixels — the same as the number of possible 24-bit colors. So this image holds every color your screen can show, each one exactly once, sorted into a rainbow: hue runs left to right, dark to light runs top to bottom. Some hues are more common in the RGB cube than others, so the rainbow stretches and squeezes.