Gold nanoparticles appear red due to a phenomenon called surface plasmon resonance (SPR). At the nanoscale, the conduction electrons on the surface of gold nanoparticles oscillate collectively in response to light, resonating with specific frequencies, particularly in the green and blue regions of the spectrum. This causes the nanoparticles to absorb green and blue light while reflecting and scattering red light, giving them their distinctive red appearance. The exact hue depends on the size of