That's really a question best left to the folks at Custom, although I suspect they won't want to endorse anyone messing with the mortar they produce. Masonry colorants are made to go in grouts and mortars, but I have never heard of using them in thinsets.
Why do you want to install your tile or stone without grout? Grout serves as more than just an aesthetic feature.
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Kitchen & Bath Renovations (VA USA)