Why is the Virgin Mary's mantle blue?

predominant color of the earth - color of the sky and the sea

It also symbolizes purity.

Answer

The above answers are often quoted but many believe them to be an urban myth. There is really quite a practical reason why Mary's mantle is always depicted as blue in art. Obviously no one knows what colour Mary's clothes were in actual life, but we are certain there were NOT blue or purple as this colour dye was particularly expensive in those days and rederved for royalty. This is confirmed at Christ's crucifixion where the soldiers placed a purple robe on Jesus and mocked him calling him 'king' of the Jews. Also, in Acts, the church leader Lydia, a wealthy merchant, dealt in purple cloth - only mentioned as it was a very unusual profession - especially for a woman.

In art, therefore, Mary had to stand out from the crowd, ordinary people that wore normal colours of brown, yellow or red, as she was seen as someone special by the church because she was the mother of Jesus himself. In the middle ages, paint pigments were obtained from either different coloured clays (like yellow and brown) or ground up minerals (like cinnabar for red or lead oxide for white). However, the only blue pigment possible in those days was a ground up precious stone called ultramarine, which cab still be found in expensive jewellery today. Nowadays the pigment ultramarine is synthetically made and is cheap, but then it was extremely expensive. In fact it was several times more expensive than gold. Therefore it was THIS pigment that was reserved for Mary's robe alone and nothing else in medieval religious art. Jesus was often depicted in gold leaf but Mary in blue ultramarine to show her importance.

In Wales where I come from there is an outdoor museum where buildings have been re-erected there to show life in the country over the centuries. One building is a medieval church that boasts a huge restored frieze around the walls of the church. The entire frieze depicts pictures from the Bible and is painted using pigments only found in medieval times. Most of the frieze is surrounded in pure gold leaf and the characters are painted in natural colours. However, there is one tiny picture of Mary in the frieze, whose robe is painted in ultramarine. The curator said that that one tiny patch of pure ultramarine paint cost more than the whole frieze full of gold leaf.

Therefore, Mary is depicted in art wearing blue as she is deemed by the early church as being very special indeed.