Forget color, go for personality and health! Yes, even in those blasted cups, bettas will display different personality traits, like aggression, fatigue, fear, and excitement.
I like to find the most active, well proportioned, smaller (younger I hope) and responsive betta. Blue and reds are both great. Get a blue with a black head (healthy sign) or a perfect red, or a mix!
Pick a strong, even body (not stubby), and nice full fin formations.