Look at Origin of gender-neutral nouns such as "la/el artista", "la/el testigo", "la/el poeta". I had the same question for adjectives also. Why do we say "un suéter naranja", and not "un suéter naranjo". And same thing with adjectives ending in -ista. Why do we not change it to -isto for masculine nouns?
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.
I will answer believing in Wikipedia like a reliable source. I qoute the next pledge of this Wikipedia reference for color genders
Solamente los siguientes colores cuando actúan como adjetivos tienen el mismo género al actuar como sustantivos que el sustantivo al que califican: blanco, negro, amarillo, rojo, rosado, anaranjado, morado y castaño.
Your specific example is about an exeption, "naranja" is not in the list of colors that change by the gender of the subject. So in both genders of subject, will be used "naranja".
On other hand, "anaranjado" will change by the subjects gender. Example:
Ella lleva una camisa anaranjada.
We use "anaranjada" because
camisa is a female subject.
Ese vaso es anaranjado
vaso, a male subject. We need to use "anaranjado".
I hope I have helped you.