When creating a custom CType the assets are missing in preview. Also it would be nice for the user to have some more values shown in preview like subheader, author, color, etc.. For this we implemented our own set of preview templates (thanks 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 color and inherit inside the child elements. If none is set, the color of the element is taken.