Tip: You can use Lunar Magic's Overworld Layer 3 Editing system to edit or create your own layer 3 images (castle bricks, fish, rocks) and apply unique effects per level via the Layer 3 Bypass in the main editor.
A tool to generate various effects for SMW or any SNES game in general.
It features a variety of easy to use tabs to create effects such as:
-Background Color HDMA
-Forground Color HDMA
The original game doesn't intend for other HDMA to be running during levels, so message boxes directly write to $0D9F instead of setting the appropriate bits. This can be fixed by hex editing $05B129 and $05B296 (see the ROM map).