- perro guía (guide dog)
- código fuente (source code)
- hora punta (rush hour)
- sitio web (Web site)
- kilowatt hora (kilowatt hour)
- año luz (light year)
First of all, a few words on invariable adjectives: The most common invariable adjectives are some colors, particularly the unusual ones. In a sense, the invariable colors are nouns masquerading as adjectives. As long as speakers think of them (probably unconsciously) as nouns, they don't change form as plurals. For example, rosa is commonly used to mean "rose colored" or "pink." As an invariable adjective, then, it is possible to say not only la casa rosa for "the pink house" but also las casas rosa for "the pink houses." But in the perception of many speakers, rosa is a regular adjective, so las casas rosas would be said. The same is true of many other colors such as sepia (sepia), beis (beige), cereza (cherry) and paja (straw). (Constructions such as color de cereza and color cereza are also extremely common, thus avoiding ambiguity).
Traditionally, the adjectives macho (male) and hembra (female) are also invariable adjectives, although many speakers nowadays use them like any other adjective. Thus you'll see both las jirafas hembra and las jirafas hembras for "the female giraffes."
Most of the other invariable adjectives are words imported from English. Examples include modelo (meaning "ideal" or "exemplary"), sport (as in los coches sport, the sports cars) and sexy. Occasionally, a translated English word is used as an invariable adjective. For example, clave means "key," not only the kind that is used with a lock, but also as an invariable adjective as in la palabra clave, "the key word."
Note that when invariable adjectives are used, the singular form of the modifying word (that is, the adjective) is used even when what it describes is plural. The same is true with the two-word nouns: When a two-word noun is made plural, the first word is made plural following the usual rules, but the second word, which usually serves to describe the first, is left in the singular form. (English does something roughly similar with words such as "father-in-law," which has the plural of "fathers-in-law.")
Generally, two-word nouns consist of a noun followed by a second noun that describes the first (the opposite of English). Exceptions to this rule include ciencia ficción ("science fiction") and coche bomba ("car bomb"), both of which copy the English word order.
Important note: While in English it is possible to indiscriminately use nouns as adjectives, the same is not true in Spanish. The use of two-word nouns is a rare exception, not the rule. You should not coin two-word nouns or invariable adjectives; you should only use ones that you have heard or seen used and know that your audience will understand.