The reason why a hydrogen lamp appears red to the naked eye lies in its emission spectrum. When hydrogen gas is energized, it emits light at specific wavelengths corresponding to the transitions of electrons between energy levels. The most prominent wavelength emitted by hydrogen falls within the red part of the spectrum, specifically at around 656 nanometers, known as the H-alpha line.
Although hydrogen does emit light at other wavelengths (including ultraviolet and blue), these wavelengths may not be visible to the human eye or may be much weaker compared to the red light. The sensitivity of our eyes also plays a role; human vision is more sensitive to certain wavelengths of light, especially in the green to yellow range, but under the conditions of a hydrogen lamp, the strong emission of red light and the lesser intensity of other colors mean that we predominantly perceive the red hue.
In summary, the hydrogen lamp emits light across a range of wavelengths, but the significant intensity of the red emission combined with the limitations of human vision is why we primarily see it as red. The other colors are either too faint or outside the visible spectrum, making them undetectable to the naked eye.