Excessive exposure to the sun causes damage to the skin and the body naturally combats and seeks to repair the damage and protect the skin by creating and releasing further melanin into the skin's cells. With the production of the melanin, the skin color darkens.
Make sure to look out for the PAs~
Personally I do not think that it is a must to use SPF 50 all the time, as long as it is PA +++ I know that I am protected from the rays that will cause damage^^
For maximum protection, go for SPF 50 PA +++
I will try to consolidate a list of my favourite sunscreens and do a post on it! Promise!