I built a castle and the stone part of the castle blends in with the mountain behind, if i use darker palette for the mountains it blends in with the rooves of the castle. Should i worry about the blending with the mountain or no?

You can change the whole pallet of either the terrain or the castle. I would change the pallet of the stone parts of the terrain to granite type blocks like bricks/granite/terracotta/rooted dirt and maybe use dripstone and packed mud for the darker areas