That may be moisture in the blocks from being stored tightly together or manufacture. Check with your installers.

Did you clean the old blocks first might make a difference to the colour ?

I did not....the old blocks are the lighter ones.

Pavers will fade over time and they get dirty and that changes their appearance also. I do think, if given time the new ones will reach a color closer to the existing. Then I would recommend you have the whole area professionally cleaned and sealed. Enhancing sealer will bring the pavers closer to their original color until they fade again.

I've had my pavers for twenty years and they haven't faded. I think that issue may be due to the type and quality of the pavers. I agree with going back to the manufacturer and ask them about fading issues.

Maybe the word "fade" needs to be defined. What I mean is wear, which changes paver appearance by exposing the aggregate bits that are inside as part of the material. All pavers have some finely crushed stone in them. The surface cement wears off over time and exposes the stone. But it's still a subtle blend -- not like a wound. It is possible that a manufacturer can use aggregates from different sources and this would slightly change the color/character of the paver as it wears, even if the paver model & color were the same.