Using the change in the intrinsic optical properties of YMg-based thin films upon exposure to hydrogen, we observe the presence of hydrogen at concentrations as low as 20 ppm just by a change in color. The eye-visible color change circumvents the use of any electronics in this device, thereby making it an inexpensive H₂ detector. The detector shows high selectivity towards H₂ in H₂-O₂-mixtures, and responds within 20 s to 0.25% H₂ in the presence of 18% O2.