When creating a custom CType, the assets are missing in the preview. Also it would be nice for the user to see some more values in the preview like subheader, author, colour, etc. For this we implemented our own set of preview templates (thanks to in2code https://www.in2code.de/en/recent/typo3-previewrenderer-preview-in-the-backend-for-your-own-content-elements/).
Inside a grid (container) we ask for the parent colour and inherit it inside the child elements. If none is set, we take the colour of the element.