Thank you for playing, and reporting me this music issue!
I'll see what I can do, and how I can fix it.
For this GameBoy-like color transition, I simply set the 4 hexadecimal colors I want as variables and then I replace colors one by one depending on if I want to make a fade-in or fade-out transition effect. A color replacement is done in approx. 0,2s to give that retro effect.
I'm using Construct 3 as game engine. I can share a tiny template with the example above if needed.