On maps, the colours displayed by Prismarine blocks and Dark Prismarine blocks are switched. This is also the case for their slabs and stairs, and Prismarine Bricks have the expected colour.

What I expected to happen was...:
I thought that Dark Prismarine would appear darker than Prismarine.

What actually happened was...:
The inverse was true.

Steps to Reproduce:
1. Place Prismarine and Dark Prismarine in the world.
2. Make a map in the area with the Prismarine and Dark Prismarine you have placed down, so that both are visible