Background flashing when Friender is hit

[Esme]

In Wood Man stage, when Friender (doggo midboss) is hit, the background tiles (the wavy pattern behind Mega Man, not the walls/floors) change to the vanilla color for exactly 1 frame.

Longer explanation copied from Discord: It looks like Friender is a mix of background tiles and sprites. When hit on vanilla, the sprites flash white for 1 frame, then the tiles flash white for 1 frame. But here's the interesting part: Friender's background tiles share a palette with the level background (the wavy pattern in empty space, not the floors/walls). So I'm guessing the game sets it and restores it somewhere, and one of those places still uses the vanilla color instead of the one set by palette randomization.

So fixing this should just be a matter of finding that alternate palette and patching in the right color.

[Esme]

Screenshots:

Unlike my faulty memory, the screenshots show that the background changes on the same frame that the Friender's background tiles change to white.

[Esme]

I found it, the background color is at offset 03A11B in the ROM. The vanilla color is 09. The following 2 values are the other colors in the alternate palette, which are 20 20 in the ROM, while the usual colors are 2C 12.