Rainbow

Rainbows were a weather formation, appearing as large arcs of colour which crossed the sky.

Rainbows typically bore the colours red, orange, yellow, green, blue, indigo, and violet, in that order. Ordinary rainbows were immaterial, being composed of light, but a rainbow emitted by a broken Filbik colour coder was solid enough for the Tenth Doctor to climb.

According to superstition, rainbows had pots of gold at the end. (PROSE: The End of the Rainbow)