You will probably be familiar with the word glas as the Welsh equivalent of English blue. However, you may notice that Welsh occasionally uses glas to refer to plants with a fresh green colour. For example, the Welsh word for living green grass is glaswellt, which combines the colour term glas with the noun gwellt (grass, straw, or hay). The other Celtic languages all use a word from the same root as glas to refer to colours ranging from fresh green to blue. Breton and Cornish both have a word related to gwyrdd, but use it only for artificial shades of the colour. This difference between Celtic languages and English in dividinig the colour spectrum explains why Welsh sometimes uses glas in idioms and compound words relating to immaturity and lack of experience, where English might use green instead. (See the entry glas for examples.)

Unlike most Welsh adjectives, gwyrdd has a feminine form: gwerdd. In informal speech and writing, many Welsh speakers don't bother with feminine adjectives, and use gwyrdd with both masculine and feminine nouns. However, you're very likely to come across the feminine forms in more formal contexts. In politics, for example, the name of the Green Party in Welsh is y Blaid Werdd.s