You can use face creams with 40 SPF .To treat dark complexion, there are several methods. Gram Flour, tomato juice, Papaya Juice, Oatmeal-honey DIY, Raw Cocoa Butter, Raw Shea butter or Raw Mango Butter, yogurt etc.
Lemon is beneficial too. But, I would not suggest to use Lemon juice directly on skin. It is highly acidic and has very low pH value. You can dilute it with water and mix honey within the solution. It is also beneficial.