Over header multicolor bar, using pseudo element :before a <header>

style.css

header:before {
	display: block;
	content: '';
	height: 5px;
	background-image: -webkit-linear-gradient(right, #b145c7 9%,#7646d6 9%,#7646d6 18%,#4a5adb 18%,#4a5adb 27%,#4a78e4 27%,#4a78e4 36%,#3a9dcf 36%,#3a9dcf 45%,#38bf73 45%,#38bf73 54%,#80c83b 54%,#80c83b 63%,#cad839 63%,#cad839 72%,#e7ba37 72%,#e7ba37 81%,#e77337 81%,#e77337 90%,#e9411f 90%,#e9411f 100%,#b145c7 100%);
	background-image: -moz-linear-gradient(right, #b145c7 9%,#7646d6 9%,#7646d6 18%,#4a5adb 18%,#4a5adb 27%,#4a78e4 27%,#4a78e4 36%,#3a9dcf 36%,#3a9dcf 45%,#38bf73 45%,#38bf73 54%,#80c83b 54%,#80c83b 63%,#cad839 63%,#cad839 72%,#e7ba37 72%,#e7ba37 81%,#e77337 81%,#e77337 90%,#e9411f 90%,#e9411f 100%,#b145c7 100%);
	background-image: -o-linear-gradient(right, #b145c7 9%,#7646d6 9%,#7646d6 18%,#4a5adb 18%,#4a5adb 27%,#4a78e4 27%,#4a78e4 36%,#3a9dcf 36%,#3a9dcf 45%,#38bf73 45%,#38bf73 54%,#80c83b 54%,#80c83b 63%,#cad839 63%,#cad839 72%,#e7ba37 72%,#e7ba37 81%,#e77337 81%,#e77337 90%,#e9411f 90%,#e9411f 100%,#b145c7 100%);
	background-image: linear-gradient(to left, #b145c7 9%,#7646d6 9%,#7646d6 18%,#4a5adb 18%,#4a5adb 27%,#4a78e4 27%,#4a78e4 36%,#3a9dcf 36%,#3a9dcf 45%,#38bf73 45%,#38bf73 54%,#80c83b 54%,#80c83b 63%,#cad839 63%,#cad839 72%,#e7ba37 72%,#e7ba37 81%,#e77337 81%,#e77337 90%,#e9411f 90%,#e9411f 100%,#b145c7 100%);
	background-size: 650px 4px;
}