A gas sensor for detection of hydride and/or acid gas species, featuring a color changing material that changes color in exposure to hydride and/or acid gas species, component(s) for impinging radiation on the color changing material for reflection therefrom, and component(s) for receiving reflected radiation from the color changing material and responsively generating an output upon a change of color of the color changing material indicative of a presence of hydride and/or acid gas species in gas contacting the color changing material. The gas sensor may be embodied in a compact and efficient probe assembly, utilizing optical fibers to transmit incident radiation to the color changing material and to transmit reflected radiation to a detector and signal processing circuitry.