Most glow-in-the-dark resin animals glow green, yellow-green or aqua — even the ones that look pink, purple or blue in daylight. Two reasons: strontium aluminate naturally emits around 520 nm, and the human eye is at its most sensitive in low light right around 500–555 nm. Manufacturers deliberately pick the wavelength your dark-adapted eyes can actually see. Blue and purple glow pigments exist, but they read as dimmer to us at night.