CSS Live Demo: text-decoration-color Click the demo button below!
selector {
    text-decoration-color: <color>
}
/* <color> values */
text-decoration-color: currentcolor;
text-decoration-color: red;
text-decoration-color: #00a0e9;
text-decoration-color: rgb(255, 255, 128);
text-decoration-color: rgba(255, 255, 128, 0.5);
text-decoration-color: transparent;

/* Global values */
text-decoration-color: inherit;
text-decoration-color: initial;
text-decoration-color: revert;
text-decoration-color: revert-layer;
text-decoration-color: unset;
<p>The text decoration color is blue</p>
p {
    text-decoration: underline;
    text-decoration-thickness: 3px;
    text-decoration-color: blue;
}
Rendered Output