Ok, I think I understand what you're trying to do. I should have specified (sorry) that you need to be editing the HTML inside your .Rmd file, not in the .html output file.
So in your .Rmd file you might have something like:
Some text.
``{r}
my_answer <- my_code(data)
a_plot_of(my_answer)
``
Some other text.
Here is some text, <p style="color:red">this part of which is red!</p>
(Please accept this missing ` around the code chunk, I had to try and format it for this message board).
When you knit this to HTML, you should see the text printed out in red. You can read more about in-line HTML styling here: https://www.w3schools.com/html/html_styles.asp