if it is getting darker, it is probably damp from water penetration, especially if there is muddy ground.
I have slabs with a good fall so rain runs off, towards the lawn which is lower, to drain away. The slabs are set in mortar so not much absorbtion from below.
My neighbour has his slabs flat, so water lies on them, and they are certainly darker and dirtier than mine.
Yours don't look brown to me.
Damp concrete usually goes a darker grey.