"J'aime une couleur, le vert."
The translation of "J'aime une couleur, le vert." is : "I like a color, green" or "I like one color, green" depending of the context.
Please note that in this exercise "vert" is a noun, as such it has its own gender (and in French the nouns of colors are masculine: le rouge, le vert, le bleu, etc...)
But in this sentence:
"J'aime la couleur verte" would be "I like the green color" or "I like the color green."
In this case "vert" would be an adjective, so you need to match it with the noun. Since "couleur" is feminine, "vert" will change for "verte".
'la couleur rouge' translates to 'the colour red' (see duolingo Basics Lesson examples for this word). The use of the word 'rouge', in that example, as a noun, does not necessitate modification of sentence structure. If that example is not a mistake, then 'j'aime la couleur rouge' should be acceptable here.