You can upgrade your survivors to an even higher rarity than Legendary and have them reach the rank of Elite, the highest rarity.
You can promote your survivors up to 10 stars by upgrading their traits. Once all the traits are on the same rarity level as your survivor, you can promote the survivor to the next rarity level; Elite.

When the survivor rarity goes over 5 stars, the stars of the survivor turn pink one by one. All the stars will be pink once your survivor has reached the maximum 10 star rarity level.

Every new pink star gives an extra 5% to the base damage and health of the survivor, and this goes into effect when the promotion is made as well as whenever the survivor is trained for the rest of the game.

Moreover, regular survivors encounter an increased chance of bodyshots when fighting enemies 3 or more levels higher, whereas a 6-star survivor will face the increased chance when enemies are 4 or more levels higher. With 7-star survivor you will again have an increased chance for bodyshots one level later, and so on.

Consider carefully when you promote your highest level survivor to Elite, since the difficulty level of hard Scavenge missions starting difficulty will go up by 1 for each pink star your survivor gains.

In the apocalypse, pink looks good on everyone!