My idea for the coloring would be to use [wrap=highlight] around the stuff you want highlighted and then apply the style to <div class="d-wrap" data-wrap="highlight">
The next step would be to add a composer button to apply the highlight to selected text in the composer, and then you could go about doing it without opening the composer.
Discourse Mermaid is a good theme component to use to see how to add a composer button. That targets code-blocks, though. https://github.com/literatecomputing/discourse-qrcode-wrap-theme-component is a theme component that I wrote that generates a qrcode using a wrap.