The correct answer is option 4, which is colorless. Hydrogen gas is a colorless gas, meaning it does not have any distinct color. This is because hydrogen gas is composed of individual hydrogen molecules, which are made up of two hydrogen atoms bonded together. Since hydrogen gas molecules do not absorb or emit any specific wavelengths of visible light, they do not have a characteristic color that can be observed by the human eye. Therefore, hydrogen gas appears transparent or colorless.

It is important to note that the appearance of hydrogen gas can vary depending on the conditions or impurities present. For example, if there are impurities in the gas, it may appear to have a slight yellowish tint. This can occur when impurities such as sulfur are present. However, pure hydrogen gas in its ideal form is colorless.

In conclusion, while hydrogen gas may appear to have a slight yellowish tint due to impurities, the pure form of hydrogen gas is colorless.