I thought Duolingo said colours should be capitalised when not modifying anything? (But many thanks for your reply.)
That's true when you're talking about a colour as an abstract entity, e.g. meine Lieblingsfarbe ist Gelb "my favourite colour is yellow".
You're not saying that you have a yellow colour -- "yellow" is not modifying "colour". It's a noun on its own.
But mein Auto ist gelb "my car is yellow", there you are talking about a yellow car. You're not saying that a car is a colour. (cars are vehicles; colours are colours.)