Isatis tinctoria
Celtic blue is a shade of blue, also known as "glas celtig" in Welsh, or "gorm ceilteach" in both the Irish language and in Scottish Gaelic. Julius Caesar reported (in Commentarii de Bello Gallico) that the Britanni used to colour their bodies blue with vitrum, a word that means primarily "glass", but also the domestic name for the "woad" ("Isatis tinctoria"), besides the Gaulish loanword "glastum" (from Proto-Celtic *glastos "green"). The connection seems to be that both glass and the woad are "water-like" (lat. vitrum is from Proto-Indo-European *wed-ro- "water-like").