Fast Blue dye showing up in blue and green channels

I am using the dye Fast Blue (FB) for retrograde labeling of neurons in the Dorsal Root Ganglia.

There is clear FB signal in the tissue. I am using a Leica epifluorescence microscope with an LED light source and various chroma filter cubes.

What I’m noticing is that I am seeing FB signal in the DAPI range using violet illumination and blue emission filter, but also using blue illumination and the green (L5) emission filter set. So both channels.

Many studies have combined Fast Blue with a green fluorophore, so I’m wondering what’s happening here.

There is no published spectrum for FB so that really limits the detective work that can be done, but I suspect that the excitation and emission spectrum for FB must be broader than just UV-Blue range. I see papers using Confocal, which will have more narrow excitation (405, 488 nm lasers) and emission filters.

Anyone ever seen this and have any thoughts?