Red

#FF0000

rgb(255, 0, 0)

hsl(0, 100%, 50%)

.element {
  background-color: #FF0000;
  color: #FF0000;
  border: 1px solid #FF0000;
}

.element {
  background-color: rgb(255, 0, 0);
  color: rgb(255, 0, 0);
}

.element {
  background-color: hsl(0, 100%, 50%);
  color: hsl(0, 100%, 50%);
}

<div style="background-color: #FF0000; color: white;">
  Content
</div>

element.style.backgroundColor = "#FF0000";
element.style.color = "#FF0000";

<div style={{ backgroundColor: "#FF0000", color: "white" }}>
  Content
</div>