"Une cocarde bleu et rouge"

Jim, 

One item of more than one colour - no adjectival agreement.

 See lesson attached on compound colour names. 

 For a nuance for multiple items, see link from Laura Lawless, section 2 b, on ‘ connected colours’.

The same multi-colours in one or several of the same ‘ thing ‘ are described invariably without adjectival agreement to the noun. With multiples of the ‘ thing ‘ having different colours to each other, adjectival agreement of colours to the noun occurs.

 Compound colour names and colour names derived from things are invariable (French Colour Adjectives)

https://www.lawlessfrench.com/grammar/color-adjectives/

Ah - so  "blue and red" is treated like "dark blue" - or maybe as if it were "blue-and-red"?