Definition of irisation or iridescense: colours appearing on clouds, sometimes mingled, sometimes in the form of bands nearly parallel to the margin of the clouds. Green and pink predominate, often with pastel shades. Irisation colors, best seen when the sun is hidden, are often brilliant and resemble those observed on nacreous or mother-of-pearl clouds. Within about 10 degrees from the sun, diffraction is the main cause of irisation, beyond 10 degrees interference is usually the predominant factor. Sometimes irisation extends to angles exceeding 40 degrees from the sun.  They occur most often in altocumulus, cirrocumulus and especially in lenticular clouds, sometimes also in a freshly formed contrail. Iridescence is seen mostly when part of a cloud is forming because then all the droplets have a similar history and consequently have a similar size

The picture was taken during an evening when the sun began to hide itself behind the mountains: the bright irisation colors became visible in a lenticular Altocumulus cloud.