The large, easy-to-read number "5" in the lower right corner on the back of the bill, which helps those with visual impairments distinguish the denomination, has been enlarged in the new $5 bill design and is printed in high-contrast purple ink.

from the US Bureau of engraving and printing on facts about 5 dollar bill: (why purple?)
• The background color is light purple in the center of the bill which blends into gray near the edges.
• An arc of purple stars surrounds the portrait and The Great Seal on the front of the bill.
• On the back of the bill, a larger, purple number “5” appears in the lower right corner to help those with visual impairments to distinguish the denomination.
• Small yellow “05”s are printed to the left of the portrait

as the why purple question seems to a be a mix of trying to be more euro trendy and for counterfit issues, seems to be a lot of public opinion about the color- some hate it (many do!) but why? some think its cheap, purple is gaudy and childish silly, unreal, fake aspect? money= not colorful. color role here.

The reason why the colour purple is used during Great Lent is because the Bible tells us that Pilate and his soldiers placed a purple robe on Jesus, just before His crucifixion. (Mark 15:16-20) "...they put on Him a purple robe. Then they said, Hail, King of the Jews!" (John 19:1-5).