Separate Markdown syntax from page styling
Portable Markdown provides tools such as headings, emphasis and code, rather than a universal font-color command. A reader’s theme or the publishing site’s stylesheet can determine the resulting appearance.
Do not assume a colored-code example changes the color of ordinary prose.
Use HTML only in a compatible workflow
CommonMark includes raw HTML syntax. In a publishing workflow that permits inline HTML and author styling, an element such as <span style="color: #24543f">A note</span> can be part of an HTML/CSS solution.
That is not portable Markdown color syntax. The renderer and host must both permit the markup and style.
Account for host sanitization
GitHub’s GFM specification explicitly notes additional post-processing and sanitization on GitHub-hosted content. A parser accepting HTML does not guarantee a hosted service will preserve its style attributes.
Use the target platform’s documented features. For important meaning, prefer clear labels and structure that remain understandable when a custom color is absent.
Keep font resources separate
Read HTML text color when you control the page’s CSS. Unicode text versus fonts explains why replacing characters also does not carry a CSS color or typeface to another platform.
Your questions, answered.
Is there a universal Markdown command for red text?
No portable command works across all Markdown renderers and hosting services.
Why does an HTML span work locally but lose its color online?
The publishing host may sanitize the markup or styles, or apply its own rendering rules.
Sources & background
Sources supporting this page’s explanations. Practical examples are our editorial suggestions.