It is usually done with something on this same gradient line. It is a gradient, but it does not blend colors because the color changes are made abruptly at the same point.
If this is exactly what you're saying you want to avoid, I think the only other alternatives would be using an image or creating several fine
Anyway, I leave two examples, one by gradient and another by thin divs.
GRADIENT
body {
margin: 0;
}
.box {
text-align: center;
position: relative;
background: #eee;
height: 15px;
width: 100vw;
}
.box:after {
background: linear-gradient(to right, #9400D3 14.3%, #4B0082 14.3%, #4B0082 28.6%, #0000FF 28.6%, #0000FF 42.9%, #00FF00 42.9%, #00FF00 57.2%, #FFFF00 57.2%, #FFFF00 71.5%, #FF7F00 71.5%, #FF7F00 85.8%, #FF0000 85.8%);
position: absolute;
content: '';
height: 4px;
right: 0;
left: 0;
bottom: 0;
}
<div class="box"></div>
DIVS
body {
margin: 0;
width: 100vw;
}
#spacer {
width: 100%;
height: 11px;
background: #eee;
}
#border {
width: 100%;
height: 4px;
}
#border > div {
width: 14.28%;
height: 4px;
float: left;
}
#border>div:nth-child(1) {
background: #9400D3;
}
#border>div:nth-child(2) {
background: #4B0082;
}
#border>div:nth-child(3) {
background: #0000FF;
}
#border>div:nth-child(4) {
background: #00FF00;
}
#border>div:nth-child(5) {
background: #FFFF00;
}
#border>div:nth-child(6) {
background: #FF7F00;
}
#border>div:nth-child(7) {
background: #FF0000;
}
<div id="spacer"></div>
<div id="border">
<div></div>
<div></div>
<div></div>
<div></div>
<div></div>
<div></div>
<div></div>
</div>