As they cure and the moisture evaporates out, they do get lighter. Not to worry

They will not get "white". They will be light grey like a sidewalk.
"not white like the ones in their picture" maybe the ones in the picture were made with white portland cement or the picture was taken in bright sunlight washing them out or maybe your screen is showing them whiter than what they are using regular grey portland cement.
If you want white use white portland cement. Billie

I guess I needed to be more patient! The first ones I made are now getting lighter:-)