What a magnificent thing to see.
Maybe you have seen it and wondered what a rainbow that high in the sky was doing. It’s not a rainbow, and it’s neither a rainbow cloud. It’s a phenomenon called “Cloud Iridescence” than happens in different types of high altitude clouds. Let Wikipedia super contributors explain it to you.
If parts of clouds have small droplets or crystals of similar size, their cumulative effect is seen as colors. The cloud must be optically thin, so that most rays encounter only a single droplet. Iridescence is therefore mostly seen at cloud edges or in semi-transparent clouds, and newly forming clouds produce the brightest and most colorful iridescence.
The cloud in question is (I believe) a Cirrus Cloud.