I was initially surprised too, but I think there is a some trick...

I'm not sure what happens on paper, but when you have ink disolved in water the abortion is not linearly proportional to the concentration, it's exponential. For example, consider a red ink and 5 magical selected frequencies and the absortions at 1% of concentration are: 99%, 99%, 50%, 10%, 1%

If you double the ink at 2% you get 99.99%, 99.99%, 75%, 19%, 1.99%

So increasing or decreasing the ink concentration may give you information of different frequencies, even with only one ink and only one sensor. In this case mostly about the 3rd and 4th. With more concentration you may kill all the light in the 3rd and measure the absortions ratio between the 4th and the 5th.

One problem I see here is how to order them, but I guess it's possible with a few sensors and a few inks. Each sensor sees all the frequencies, but with different weight. I'm not sure if it's possible to solve this, but perhaps you need some initial approximated model of the inks(???).

Now, when you put the inks on paper and have a unreliable light source and perhaps other technical problems, ...

In conclusion, I think it's possible to use different saturation and mixes of the inks to get different spectral distribution of the light that bounce on the card. Then use the three sensors to get three averages and try to use big linear algebra book to reconstruct what happens in between. But I should read the paper to be sure.