Colors derived from objects and how they match nouns
In Portuguese, adjectives normally change to match the noun they describe. A plural noun gets a plural adjective, and a feminine noun gets a feminine adjective. However, color words can break this rule if they are derived from the name of an object.
Words like rosa, laranja, and cinza often ignore normal rules for matching nouns. They can keep their original singular form no matter what noun they describe. This can feel strange to English speakers, but there is a very logical historical reason for it.
The history behind this rule is simple. When you describe something as orange, you are essentially saying it has the color of an orange fruit. In Portuguese, this was traditionally expressed with the phrase cor de, meaning color of.
Because you are referencing a single fruit or flower, the phrase does not change if you have one shirt or five shirts. Words like cor-de-rosa and cor-de-laranja are perfect examples of this. They never change their form to match a plural noun.
Today, speakers usually drop the cor de part and just use the noun itself as an adjective. You will frequently see rosa, laranja, cinza (which originally means ash), and púrpura (a purple dye from sea snails).
According to traditional grammar, the color of idea is still secretly there. For this reason, these words often do not change form. They stay singular even when describing plural nouns.
Languages change over time. Words like cinza, rosa, and laranja, along with other borrowed colors like bege, have been used as adjectives for so long that they are starting to follow normal adjective rules.
It is very common, especially in everyday Brazilian Portuguese, to see these words pluralized to match the nouns they describe. You can choose to use the traditional form that does not change or the pluralized form. Both are recognized in modern usage.
Just remember that while the single-word colors can be pluralized, the compound words cor-de-rosa and cor-de-laranja always stay exactly the same.
When you use a color name that comes from a physical object, it is perfectly correct to leave it singular even when describing plural nouns. If you use a compound word starting with cor-de-, it never changes form at all.