I used Ban microflex grout and then sealed it with a grout sealer.
Behind the tile is tile backer board.
The shower tray should be sealed with mould resistant sanitary silicone to the backer board when it is first fitted and then once you have finished tiling and grouting you then silicone around the silicone tray again so it then has a double seal, it is the same principle with a bath. The colour of silicone you use depends on the colour grout you choose so it matches. It is always good practise when you silicone around the bath to make sure there is weight in the bath until the silicone has cured otherwise the silicone bead will split at a later date from slight movement when the bath is filled up and emptied etc