Yes, I was referring to the spectrometer. Non black ink on non black paper. Sorry if I was a little unspecific. This is my first time doing anything like this.
No problem -- i guess, so for example, I'm not sure if red ink shows up on black paper, but we use colored ink (red, blue) on brown paper when we make the ones in the store: https://store.publiclab.org/collections/spectrometry
That seems to work pretty well, especially when we print black onto the back side.