Site para alterar a cor degrade: https://cssgradient.io/
Script
.head-primary::after {
content: "";
display: block;
background: linear-gradient(to right, #ffff4a, #fcd000, #ffc112, #ffc112, #ff8a00, #ff5f5f, #ff253a, #ff37a8, #c739ff, #a400e1, #2eceff, #0086ff, #72f772, #00d604);
height: 5px;
width: 100%;
position: relative;
}
Esse tutorial foi util?