White mealies are mealies that recently shed. Basically means it is growing. You'll notice when they are about to turn into pupae, because they will have a darker, rougher look to them, and you might be able to see weird ridges on them. Until they turn into pupae, keep them in with the rest.
Well, dead mealies turn black, and dead pupae are dark brown and hard. Live pupae are white and turn a beige-ish color, and live mealies are white or golden. Mealies just pupating will be a darker brown because they are about to shed a large piece of skin, and underneath is the white pupae.