Probably not - the material preview is basically a light box based (hri image of unreal HQ) system.
It includes reflections and a bunch other things that are just not avaliable and should not be avaliable in your final scene.
That said you could also have other issues going on.
The material preview is set up with default values for the painted layers. The weight of each layer is selectable in your left hand panel.
Thus, iit’s possible you have a shade set to 50% therefore what you see on the landscape is accurate vs what the material preview shows you.
If you are choosing colors into an instance, its generally desired to view the resut in Pie and go off of that instead of focusing on the material preview…