The best way to tell if a mango is ripe is by the feel of it. Give the mango a gentle squeeze and it should give slightly which means that the flesh inside is nice and soft. It is similar to checking if a peach is ripe. Don't focus on the colour too much, as different types of mango have different colours when ripe. Whilst you might think that a darker red, rather than green, would mean it is ripe, it doesn't necessarily work that way.