do u use any acne medication or topical etc?

if yes,then it makes ur skin(face) photosensitive.

so u need a minimum of spf 30 applied generously.

my face and forearms are darker than my whole body lol

You're suppose to match your foundation to your chest. SPF 20 is low.I use SPF 100 and I never have to worry about getting darker.

the part open(unclothed) is anyway likely to be quite darker.

thats not uncommon. like the face/neck, hands, feet/lower & inner part of the limbs etc.

my neck is slighly pigmented/darker over all (not from acne. and i dnt think its from the sun as well).sometimes its genetics i suppose.

am unsure.

the slight darker colour starts from the lower part of my face(jawline) and extends upto my neck.

u dnt need spf 100 to protect yourself.

spf 30 is enough.

read acne.org's suncreen page for detailed information.

My face is darker than my neck too, especially around my chin where I have redness/PIH. My neck is also more yellow toned so there's a pretty big difference haha. I usually try to match my foundation to my neck.