I've mixed up gallons of black epoxy over the years. I lucked into old school toner from Xerox Printing for nothing way back - carbon black. Lee Valley Earth oxides work just the same. Not complicated or expensive.
I use Mixol #1 for black. Seems fine.
Remember, epoxy is a structural adhesive used in many stressing applications, aircraft for example, so the manufacturers frown on adding anything that might possibly lessen its strength and bonding ability. But, unless a coloring additive prevents the epoxy from curing properly, there is absolutely no reason you can't use just about any coloring medium with epoxy to fill knots, holes, etc.
Last edited by Alan Schaffter; 02-22-2019 at 10:47 AM.