Certain colour adjectives are invariable—that is, they never change regardless of the gender and number of the noun. All of these adjectives are derived from nouns.
Take orange for example. As in English, in French orange refers to both the colour and the fruit (une orange ). Though you can certainly have de multiples oranges (multiple oranges), the adjective form of the word never changes, even in the plural. This applies to all colours that are derived from nouns.
Liaisons are forbidden:
Before and after et ("and").
After singular nouns (including proper nouns and names).
After inversions (which you'll learn in "Questions").
Before an aspirated H (e.g. héros - "hero").
After a nasal sound, except that un, on, and en do liaise.
----from tips and notes in “Phrases”
Maybe this situation belongs to 3.
There are some colors which change form for masculine/feminine and singular/plural. Others are invariable in regard to gender but not to number. Still others are invariable for both gender and number. Here is a link that will tell you everything you need to know about it. https://www.thoughtco.com/french-adjective-of-color-1368982
I can't remember rules about this stuff, which always seem to have so many exceptions that it's impossible to make a rule. All I take away is like determining the gender of nouns, or spelling -- "Just keep practicing until the right option is clearly embedded in your brain and the wrong one feels weird."
Never. Some BrE speakers use "has got" instead of "has". The FR "des" is the plural of "une". In an effort to translate each and every French word into English, some people use "some" but it is unnecessary and is almost always ignored in English in this context. https://www.thoughtco.com/du-de-la-des-1368977