blue
noun
the pure color of a clear sky; the primary color between green and violet in the visible spectrum, an effect of light with a wavelength between 450 and 500 nanometers.
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If it is desired to give the paper a white shade, a small amount of aniline blue or pink is mixed in; otherwise it is called "natural" or "unblued."
From The Building of a Book A Series of Practical Articles Written by Experts in the Various Departments of Book Making and Distributing by Frederick H. Hitchcock