I have made multiple attempts at colored inlay with 3D printing and CnC milling in the past. My best results have been with epoxy resin and dye. It is hard to work with. Either you slop it on or pipe it in with a syringe. Slopping it on runs the risk of voids and has a lot of surface cleanup. Using a syringe has better control, but the stuff kicks off quickly when placed in a container like that. I had experimented with Acrylic paints in the past with only modest success. I tried again on my 3D prints and had a bit of a breakthrough.
I picked up some craft acrylic paint for general purpose use. It comes in a bottle and is the consistency of ranch dressing. The previous stuff I worked with was in a tube and was very thick I poured some black into a 3cc syringe and piped it into a cavity of my 3D print.
I started with a 25 gauge straight needle, but the paint flowed too freely and I ran over on one of the parts. I dropped down to a 30 gauge needle and had a lot more success. The paint flows in, sticks well to everything, and doesn’t appear to run. You can use the needle to guide it up to a wall if it didn’t quite reach everywhere.
These prints are my corner radius templates for the router table. The text is face up and have 3 perimeters. Not sure if it will bleed through 2 perimeters or not. I suspect if you had inset text on the side of a print it would seep out and run along the Z-layers. I haven’t tried it yet though. If you do happen to run over, the paint wicks down into the top layer and never comes fully out. Sealing it with a spray lacquer, before you paint, might fill the gaps and buy margin for error. I was just really careful and used a small needle after the first screw-up.
Once I got the hang of it I didn’t make any more mistakes and was able to fill them out in a minute or two each part. Maybe some day I will get a multi-color capable printer, but until then, this is a really easy effective method of adding color to inlaid text on 3D prints.