All mosaic is normally grouted, except Smalti. This is to frame the Tesserae and to seal the mosaic from water and dirt which will damage and shorten the lifespan.
The thickness of the gaps become lines once grouted. Neat placement will provide nice even lines.
Many colour can be used. The colour changes the entire look of the mosaic. Here a re a few examples of the same mosaic, with different colour grout:
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