Functions

rgb(), rgba()

h1 {
  color: rgb(255, 0, 0);
}

p {
  color: rgba(255, 0, 0, .5);
}

<h1>Lorem ipsum dolor</h1>
<p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Id, quia...</p>

Output:

Lorem ipsum dolor

Lorem ipsum dolor sit amet consectetur adipisicing elit. Id, quia...

References