Because you normally write in Markdown my idea was to use plain HTML like this:
... <u style="color:red;">text in red</u> ...
And indeed you can use tags like <i> or <b> here in the text but the addition of a style attribute seems not to work. I used the <u> tag here as example but it does also not work with <p> or <span> or <code>.
Markdown itself seems not to be one agreed-upon standard, and then which HTML tags a particular forum will allow additionally is another matter.
If it doesn’t work here, it doesn’t work here.