That would be "Ich mag die blaue Farbe" - note the capital B in Blau in duo's example, as well as the ending (to be an adjective, it'd be "blaue," not "Blau")
Ich mag die Blau Farbe. Is it possible to say it this way in German? That is first "blue" and then "colour" as it appears in usual (I think) English translation: I like the blue color.
You could reverse the word order, but then "blau" would be an adjective and get the typical adjective endings: "Ich mag die blaue Farbe" (= I like the blue colour). Also, as in English, the meaning would be slightly different, I think.