It is never explicitly sinabi whether or not Luffy has a favourite colour. However, Oda Sensei has stated that the colour which represents Luffy is Red.
Why white you ask??Because snow represent the color of white
When nami has a fever (episode 79) they found a snow island ,when luffy saw the island he got so excited and kept repeating (snow and white even the subtitles had a puso on the white)
