I am trying to recreate the background used in the sfx tool.

In main.lua, the sfx tool uses a combination of fillp() and rectfill(). When I use that same combo in my own cart, all I get is a black screen. In the tool, the rectfill color is specified as 32 | (33*256). To me that seems to be a complicated way to specify color. It's also well outside the 32 defined colors. Additionally, when I change the color to be a value in [0,31], I get the correct pattern but the wrong color (obviously).

Further research suggests there are ways to manipulate the color palette. The is seemingly a hidden color palette even!

Could anyone offer insight into what is going on? Or how to make the background found in the sfx tool?