Numbers 15:38

That they put upon the fringe of the borders a ribband of blue -Reader that they add to the fringes of the borders (or corners) a thread of blue (compare the marginal references). These fringes are considered to be of Egypttian origin. The ordinary outer Jewish garment was a quadrangular piece of cloth like a modern plaid, to the corners of which, in conformity with this command, a tassel was attached. Each tassel had a conspicuous thread of deep blue, this color being doubtless symbolic of the heavenly origin of the commandments of which it was to serve as a memento. Tradition determined that the other threads should be white - this color being an emblem of purity (compareIsaiah 1:18).