Blue and Purple paint and dyes were considered royal and very expensive because of their scarcity.
Re: Purple
Purple dye was the most rare in ancient days. Purple fabric used to be so outrageously expensive that only rulers could afford it. The dye initially used to make purple came from the Phoenician trading city of Tyre, which is now in modern-day Lebanon. Fabric traders obtained the dye from a small mollusk that was only found in the Tyre region of the Mediterranean Sea. A lot of work went into producing the dye, as more than 9,000 mollusks were needed to create just one gram of Tyrian purple. Since only wealthy rulers could afford to buy and wear the color , it became associated with the imperial classes of Rome, Egypt, and Persia. Purple also came to represent spirituality and holiness because the ancient emperors, kings and queens that wore the color were often thought of as gods or descendents of the gods.
https://www.livescience.com/33324-purple-royal-color.html
Re: Blue
In early antiquity, the first blue pigments were produced from crushed gemstones such as azurite and lapis lazuli. So valued were these gemstones that an old Persian legend stated that even the sky was blue because the world was supported on a huge chunk of lapis lazuli.
Making these pigments was a very costly exercise as in ancient times lapis lazuli was mined in the high mountain passes of the Badakhshan region of Afghanistan. It then had to be transported great distances by camel train to be traded with the flourishing civilisations of Mesopotamia, Egypt, Turkey, Greece and even deep into Africa. ….
Blue in Ancient Egyptian Mythology
The use of colour has always been highly symbolic and in ancient Egyptian mythology blue was associated with the sky and the waters. Blue is the colour of the sky and represented the male principle, the sky deities, and the gods of heaven.
The depths of the deep blue waters represented the female principle and the deeper, hidden mysteries of life. It was believed that the very hair of the Egyptian gods was made from vivid blue lapis lazuli.
The great Theban god Amen was known as the hidden one and he could change the colour of his skin to blue so that he would be rendered invisible as he flew across the sky. Blue was associated with life and rebirth as the world was said to have risen from waters of the primeval floods on the day that the sun rose for the very first time.
https://owlcation.com/humanities/The-Colour-Blue-in-Antiquity