Resin is made up of combinations of oligomers and monomers that are photosensitive. In the context of additive manufacturing, this photosensitivity is used in vat polymerization, where the specifically-designed resin is cured (hardened) by exposure to UV light. In the case of stereolithography (SLA), the light source is a high-powered laser, while in newer methods like digital light processing (DLP), a projector screen is used.
To some extent, these processes are comparable with fused deposition modeling (FDM) 3D printing, as a part is still created layer by layer. However, with resin 3D printing, the build plate is slowly raised away from the vat of resin as each layer is cured, rather than layers being extruded on top of each other.
Resin can be purchased in a variety of colors, but as with anything, makers have begun to experiment with other methods of coloring, including using dyes intended for food. But don’t be fooled: Even if you can add food coloring to it, resin is still toxic! When handling or printing with resin, we recommend wearing safety goggles and nitrile gloves, and always work in a well-ventilated area.
A few years ago I had to do some experiments with finding a lacquer that reflected Infra-Red light. I tried several different brands, all of them clear to the naked eye, but they were hugely variable in their response to Infra-Red.
The lesson here is that, while food colourings may seem like a good idea, you need to choose them based on their transparency to the UV light used in resin printers. Needless to say, you aren’t going to find a data sheet on this so someone will have to experiment.
The problem is: what if the food colouring absorbs UV? If it’s absorbing the UV, that means there’s less UV left for the resin, which may prevent it from curing. Of course, there’s also the questions of whether the molecules in the food colouring will react chemically with the resin and either prevent it from hardening or change the rate at which it hardens, and the question of whether the presence of food colouring molecules getting in between the resin molecular chains and preventing them from sticking to each other - kind of similar to trying to pain on a dirty wall.
I would suggest whoever tries this, try doing it with a few drops of resin mixed with droplets of food colouring added with a pin and deposited on a labelled card. You could put rows of dots of different food colourings, or different ratios of resin to food colouring, then put the car into an empty vat and apply the curing UV to see which dots harden and how long it takes.
I would definately NOT commit an entire batch of resin to one application of food colouring.
My experience with the lacquer (actually, nail polish) was that different brands may look the same, but have different properties, so you may have to try several brands of red food colouring to find which ones work, or that none of them work, or that all of them work.
I suspect that, to be thorough, it would be a tedious experiment and need thorough documentation to determine which brand/colour/ratio/UV-exposure works best.
Too many variables. My brain hurts.