Blue color or pigment; resembling the color of the clear sky in the daytime WordNet 3.0 & Open English WordNet

Blue is one of the three primary colours in the RYB colour model, as well as in the RGB colour model. Wikipedia

Color. Wikipedia Disambiguation

Primary colour between purple and green in the spectrum Wikidata

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 nm. OmegaWiki

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. Wiktionary

Colour. Wiktionary (translation)

The quality or characteristic of being blue in color. Wiktionary

Quality or characteristic of being blue. Wiktionary (translation)