Add default colour

Currently, all non-coloured items are white/empty by default until the user interacts with them and adds a colour.

One user said that they would like to colour objects beforehand, so subjects see a red car when the image appears (and can change its colour, if needed).

The issue names no files, tests, or entry points, so first inspect the repository structure and find where non-coloured items are initialized and rendered. Confirm how a preset colour is represented and verify that objects can appear with a default colour while users can still change it.

Written by the indexing model from the issue text.