The standard rule for constructing the comparative form of adjectives is the following.
If an adjective has one syllable (monosyllabic), the suffix -er is added.
For example, red ➢ redder, pink ➢ pinker. If the word ends in -e only the letter -r is needed:
e.g., blue ➢ bluer.
If users are interested, this form is called the synthetic comparative
Adjectives that have two or more syllables (disyllabic), usually form the comparative by adding more; e.g., purple ➢ more purple, turquoise ➢ more turquoise, and indigo ➢ more indigo. In standard English, the word orange has two syllables, /ˈɒrɪn(d)ʒ/, which means the comparative form is more orange.
In linguistics, it is called the analytic comparative
I said in standard English, but there are many two-syllable words in the English language that also add the suffix -er.
For example; narrow ➢ narrower, quiet ➢ quieter, simple ➢ simpler, and orange ➢ oranger.
Wiktionary lists the comparative form oranger and defines it as
comparative form of orange: more orange
Therefore the superlative can be either orangest or most orange. However, in more formal writing, I would recommend the OP use the latter.
Online I found the following examples of oranger
(a). No. 1 is redder than no.2, though both are sort of orange.
(b). 2 is oranger than no. 1, though both are sort of red.
For Bronnenkant, "A is always yellow, but Y is an oranger yellow.
- It turned out to kinda be like egg tart (only the eggy bit) but mucho sweeter and kinda oranger.
- The buckwheat is oranger than ever. You can see the shiny tin roof of Henny's shack below us, all rosy.
- “Well, your hair's oranger than an orange. You glow like a neon sign.”
As to why, @Shoe cites the following excerpt from Pratical English Usage (p114) by Michael Swan, in their answer
With many two-syllable adjectives (e.g. polite, common) -er/-est and more/most are both possible. With others (including adjectives ending in -ing, -ed, -ful, and -less), only more/most is possible. In general, the structure with more/most is becoming more common.
For more examples of disyllabic adjectives that take the suffix -er, see this old question of mine: Conundrum: "cleverer" or "more clever", "simpler" or "more simple" etc
Meaning and Grammar: An Introduction to Semantics
Leaping Man Hill by Carol Emshwiller
The Toonies Invade Silicon Valley by Betty Dravis
How speakers select synthetic and analytic forms of English comparatives:
An experimental study by Naomi Enzinna
The English comparative - Phonology and Usage (Stanford University)