The answer by brazofuerte to the question you link also applies here. Basically, it is mostly words that etymologically end with "-o" (and maybe "-ón") that change to "-a" to form the feminine. Words that etymologically end with other vowels are usually invariable. As brazofuerte's answer says, the three most common invariable suffixes are "-ista", "-ente", and "-ante", but there are many others.
As that answer also says, some of the words ending with "-ente" have developed a feminine form ending with "-enta", such as presidente, asistente, but this seems to only be valid when they are used as nouns. When they are used as adjectives, as you can see in the DLE entries, only the ending "-ente" is accepted:
- Las mujeres asistentes a la reunión elegirán a una representante.
Regarding naranja, some colors have a special treatment. If they come from nouns that designate things that have that color (such as naranja, violeta, marrón), then they can be used invariably without changing in the feminine or plural forms. See this question for more information on colors without a feminine form.