Thanks lol.
It's a bit less elegant than you might think actually. I made a gradually less transparent pngs of a rectangle in Photoshop. Then, I used a program to make each of those rectangle into 1 bit rectangles, maintaining their semi-transparency. These, newly 1 bit, rectangles were then assembled into a ~20 image sprite for the game.
All of the other graphics were also 1 bit sprites, so there was no color filtering on the games end.
I used Gamemaker Studio 2, so this was the best solution I could imagine working in such a short time. Brute force does seem to work sometimes lmao.