I purchased some liquid Rit Dye ("Chocolate" color for synthetics in my case) from Amazon; it's intended to change the color of synthetic fabrics. Since synthetic fabrics are basically plastic, I figured it might work out okay to dye my 3D prints, which are also plastic. This kind of simpleminded reasoning was all I needed to give things a shot.
Testing consisted of simply filling a plastic cup part way with very hot (but not boiling) water. Then I poured in a small amount of the liquid dye; I didn't measure anything, but it was probably a couple of teaspoons.
Then I dropped in a test piece, which was a scrap base and support structure. I stirred it a few times over five minutes, then removed and rinsed it. The result is shown below on the right.
|Plain Grey FormLabs resin on the left. On the right is also Grey resin, but after five minutes of dyeing.
I decided to do a production piece. Five minutes seemed plenty so I did four minutes. The results are below. On the left are regular prints, on the right are the dyed pieces.
|Left: undyed Grey FormLabs resin results. Right: results from four minutes in dye solution.