From Tim comes a fun site rawcolor.com which includes a page on using cabbage, beet and pumpkin juice as colorants in an inkjet printer.
What fun! But why stop with some test prints - what exactly does the spectrum of a beet look like?
Start with a beet, a file, a damp paint brush and piece of multi-function office paper. Then use a preferred cutting implement to cut the beet. Begin filing the beet until a good amount of thick juice accumulates on the file. Use the brush to apply the beet juice to the paper. As a short cut to varying the concentration use multiple layers of beet juice to get a more saturated color. Wait for your test page to dry.
Then its time to get out your spectrophotometer. I used a Gretag-Macbeth eye-one. Finally plot the spectrum of your beet.
Yes the data above has the optical brightner of the paper evident around 400 nanometers but the interesting bit is off to the right. The maximum beet reflectance is somewhere out past 730 nanometers. Makes me want to measure some other edible colorants...