Hello
Either the same colours that were on the switch ( blue and brown) or black, would not take white

I understand your arrangement to be that your shelly is installed directly in, or behind, the switch.

Essentially, the same answer applies as roland.schacher has already given: use the same colours.
For the few centimetres between the switch and the shelly, however, I don't think the colour is so essential, as the course or use within the small (surface-mounted or flush-mounted) box is immediately apparent.

However, if you are laying a new cable through a tube, for example, I would definitely use only the appropriate colour.
General information on colour use, today and potentially in old buildings, can be found e.g. here: https://diybook.ch/bauen-reno...