Do you know what it means that the colour of a filament is natural? It means that a colour additive has not been added in its manufacturing process.
In Smart Materials 3D you can find these filaments in the Smartfil range and in the Innovatefil range in natural color:
Some filaments that have a certificate, when they are coloured lose that certification. It happens with PETG and Medical, if you add colour, they lose the USP class VI or ISO 10993-1 certification, certification that guarantees the filament is biocompatible with the human body.