How to colour only a single (bottom plate) layer of a part

If I colour the face of a part, then Bambu Studio is colouring 4 layers, the first all one colour and then subsequent layers get two perimiters extra of main colour.
i.e.

  1. Face all one colour
  2. Layer 2 with two perimeters
  3. Layer 3 with four perimeters
  4. Layer 4 with six perimeters
  5. Layer 5 with all main colour

I was expecting only the face to be painted saving filament and time, not the internal as well.

If only painting the bottom layer of a part then I should also only need 1 poop and perhaps one layer or not of purge tower.

I also tried extruding some “tools” in Fusion 360 that were one layer thick. Then used those tools to do a cut but also keep tools. Then imported as a single stl into Bambu Studio, and coloured the “tools/pattern” this worked to some extent, but perhaps there are some bugs in Bambu Studio, the bottom layer had some gaps in the preview, so used Wall generator Arachne which did help with this issue before but still left gaps. And when viewing what parts were coloured
Layer one was coloured correctly, layer 2 was main colour only, but had some weird artefacts. Layer 3 was coloured but with less coloured perimeters like mentioned above and the rest of the layers were all the main colour.