The Celtic spectrum different to ours. Theirs was based on and described by the quality of the hue, not the wavelength.
Thus, the early Welsh ‘llywd’ can mean brown (like paper), blue (like mould) or grey (like rabbit) because they are of similar hue not colour. Similarly, ‘glas’ can mean grey (like a horse) or clear (like saliva). ‘Coch’ means brown (sugar) or red (meat). This spectrum trend is also found in the traditions of Scotland.