In the sentences before the one in question, the speaker is referring to how nebulae are classified based on the composition of gas particles. This is achieved through astronomical spectroscopy which uses the visible light spectrum to determine which particles are emitting light. By glowing "in the red" part of the light spectrum, they can infer that hydrogen likely makes up some of the nebula.
Use of the definite article here is due to the implied clause referring to the visible light spectrum. It could be editorialized to include the extra clause, e.g. "hydrogen glows most strongly in the red [part of the light spectrum]." An alternative way to say it would be that it simply "glows red."
The second sentence does not use the definite article because it refers to an ambiguous article of clothing, as opposed to something unique, e.g. the visual light spectrum as used in the first sentence. It could be written as "Kate was dressed in black clothes that day" to illustrate that "black" describes the property of an arbitrary article of clothing rather than a specific one.