Inline CSS is the answer you're looking for, and is generally not good practice to use consistently, as it creates a lot of unnecessary clutter and mess. Prefer external CSS, or at the very least internal CSS.
Inline CSS is when you modify the element itself, rather than selecting classes, IDs and elements like you would with internal or external CSS.
The follow HTML and CSS would cause the paragraph to have red text:
<p>
Hello World!
</p>