Grout has no adhesive qualities at all. It is meant to fill the spaces between tesserae to provide protection from the elements only, not as a glue.

Thinset, however can be used as both glue and grout. Think stepping stones where the tesserae is pushed into the thinset and the thinset is allowed to come up the side of the tesserae to form its own "grout."

Great! That's what I needed to hear! Colored thinset! (I thought choices were only neutrals (white, off-white, gray...)

Great! Thank you!

The pre-mixed thinset/grout/adhesive mixes I have used have been awful. That mixture failed on me regularly, it was a bear to clean off tesserae, and it slumped something fierce - sometimes weeks after the fact.

Just so you know what could happen.

yeah, I've got the "don't-use-pre-mixed" mantra embedded in the brain.

and that would have been me! lol

thinset is definately more difficult the clean off.

Color choices are limited and you need to mix your own colorants in to get better variety.