Our customers have had most success using water based tints, found at any paint store. Just a few drops per quart seems to do the trick. We can't guarantee the tint's lifetime as that's not our product, but the treatment itself penetrates right into the wood so it will never peel or flake.
We always recommend doing a patch test to see what the tint will look like along with the natural stain of Eco-Safe Wood Treatment or Weathered Wood Finish.
While it may be possible to add other sorts of colorants to Eco-Safe Wood Treatment or Weathered Wood Finish, when a colorant is added to the mixture we can't be sure that the treatment is penetrating into the fibers of the wood and covering all surfaces for maximum effectiveness.
Since a colorant could potentially interfere with the process, we often recommend using paints, stains, and other finishes overtop of the treatment.
For more, check out our article on using paints, stains, and other finishes.