Black dye is the only color I would do. Rit dye. Follow the directions.
Make sure the scales are scrubbed clean. No bare fingers on them after cleaning.
I use the liquid black rit dye. Follow the directions. Get the liquid hot in a pot on the stovetop. Turn off the heat and put the scales in. Let them soak for 10-15 minutes submerged. Check. Shouldn’t take more than 20 minutes to get color.
If the color isn’t taking then the scales are too polished or coated with something.