Passion, apples, blood, fire and it's the first color we see?
Passion is an obsession a type of love and is usually intense when put to the test.
Apples are associated with knowledge, the fruit of seduction all though there is no real proof, it was the forbidden fruit, most people think of it as the one given to Adam.
Blood is vital for life and goes in and out of the heart, used to express our love for one another and the blood of Christ.
Fire although has many colors, is always associated with fire the flames of love.
I'm not a 100 % on this but I also think it's the first color that children can see because of it's brilliant array.