I am just learning about Grease Pencil, and mine always appears see-through, both in the viewport and when rendered. I do not know how to fix this. Here are screenshots of the viewport and render respectively:
The default Strenght for the grease pencil is 0.600.
To draw dark strokes set it to 1
Additionally you can use an opacity modifier to make existing strokes darker.
Read: Edit Grease Pencil Strength?