"I have a pale blue shirt."
Translation:J'ai une chemise bleu pâle.
When you use more than one adjective to designate a single color (like "light blue," "dark green," "pale pink" etc.), neither of the adjectives changes according to the noun it modifies. For example:
"Il a les yeux bleu clair et les cheveux brun foncé."
"He has light blue eyes and dark brown hair."
Is this just relevant to colours? If we say, "blue shirt", the word 'blue' is an adjective. If we say, "pale blue shirt", because 'pale' qualifies the adjective, 'blue', it is an adverb. The general rule in French is adverbs are placed in front of the adjective they qualify, e.g. "je suis très heureux”, where ‘très’ is the adverb.
An adverb is not limited to being a modifier of a verb. An adjective that modifies another adjective becomes an adverb.
The distinction is important in many languages. In English, it means that you don't place a comma between them if you want the adjective to modify another adjective. If you do place a comma you are choosing to shift the meaning because the adjectives no longer modify each other.
A bright blue car without the comma is a car that is blue but the shade of the blue is regarded as bright. Bright is an adverb.
A bright, blue car with the comma is a car that is bright. The comma means that bright does not apply to blue but the car instead. Bright remains an adjective modifying the noun car. Perhaps it is in a bright light compared to other surrounding cars. It is also blue.
Marko246521 amended his comment to indicate that he meant that pale was an adverb in the sentence quoted. His original comment made it seem like he was saying blue was an adverb. Pale is an adverb as he states. (English speakers should beware the casual use of pronouns. You know which word is being replaced but it is not always apparent to the reader/listener.)
It is common in some languages for adverbs that are actually adjectives modifying other adjectives to sometimes conform to rules applied to adverbs and sometimes behave as adjectives. Since adverb/adjectives that qualify colors are a method of classifying the color, they follow the word they modify, in French.