Welcome to the Bunnings Workshop community @ffr. It's fantastic to have you join us and many thanks for your question.
If you are concerned about the dirt washing out and staining your tiles then I would recommend drain matting being laid in the bed first and the soil placed on top. The drain matting will filter out the majority of debris and minimise the staining to your tiles. You might want to consider not using fertiliser like Seasol that have a distinctive colouring as they too could stain the tiles.