Well, I tend to connect preferences and likes and such with colors. Meaning that, when I get excited about something, I'll generally picture the event in my brain as a bunch of bright colors like purple and green and such (it's weird, but whatever), so I think that's why the rainbow cake looks tastier to me.
http://www.betterhomes-cake.com/products/cake_RainbowCakeChocolate.jpg
*mariachi playing*