The colour of the clear sky or the deep sea, between green and violet in the visible spectrum, and one of the primary additive colours for transmitted light; the colour obtained by subtracting red and green from white light using magenta and cyan filters; or any colour resembling this.

A blue dye or pigment.

Any of several processes to protect metal against rust.

To make or become blue.

To treat the surface of steel so that it is passivated chemically and becomes more resistant to rust.

(laundry) To brighten by treating with blue (laundry aid)

Of the colour blue.

“the deep blue sea”

Depressed, melancholic, sad.

Pale, without redness or glare; said of a flame.

“The candle burns blue.”