In an Irish history called Cogad Gáedel re Gailab (The War of the Irish against the Foreigners) which documents the battle of Clontarf in 1014 of the forces of Sitric Silkenbeard against those of the Irish high king Brian Boru, the author uses a famously long and hyperbolic string of adjectives to describe the Vikings in disparaging terms.
Now on the one side of that battle were the shouting, hateful, powerful, wrestling, valiant, active, fierce-moving, dangerous, nimble, violent, furious, unscrupulous, untamable, inexorable, unsteady, cruel, barbarous, frightful, sharp, ready, huge, prepared, cunning, warlike, poisonous, murderous, hostile Danars; bold, hard-hearted Danmarkians, surly, piratical foreigners, blue-green, pagan; without reverence, without veneration, without honour, without mercy, for God or for man.
What did the term "blue-green" mean in the context in which the author uses it?
Gormglasa. Gorm and Glas could be translated as simply as 'blue' and 'green' and it's not wholly unreasonable for the author to have intended such a direct meaning...possibly as an invocation of the sea? George Calder alternatively offers 'dark grey' from the same phrase in Togail na Tebe which would also track here. However, I would say the meaning is fairly overt in context of the text, that being a poetic way of calling them foreigners: c.f. na nGall ngorm, 'the swarthy Norsemen'; cú glas 'foreigner, stranger' (lit. 'grey wolf'); also glasfhine referring to children of Irish mothers by foreign fathers. Some suggestion might be made towards referencing their arms and armour also, cf. cruaidh gurm 'blue steel'; gormlann 'burnished blade/armor'; crenaid claidbiu glassu 'he buys blue swords'. These are both words that saw a lot of traction in describing the Norsemen. In her Early Irish Satire, Roisin McLaughlin comments on the invective employed against the Norse:
The Early Irish metrical tracts contain references to foreigners in various contexts such as praise, satire and occasional poems, and the term gall in some of these instances may refer to Viking settlers. It is stated in MV II SS 128, for example, that: rofersat Gaill grafainn fornn / atbath ar tonn indmais 'the Norse have won the race on us, our wave of wealth has perished' (Meyer 1917, 42), while no. 44 below describes its subject as a glas cam / for gáir gall 'you crooked bolt at the battle-cry of foreigners'.
In any case, the original text provides some context:
Batar, imorro, dun darna leith in catha sin
glaim glonmar, gusmar, glecach, galach, gnimach, gargbeoda,
duabsig, dian, demnietach, dasachtach, diceillid, docoisc, dochomuind,
becda, borb, barbarta, boadba,
ath, athlum, anniartacha, urlam, angbaid, irgalach,
nemnech, niata, namdemail
danair; dana, durcraidecha,
anmargaich, anbli, allmarda
gaill, gormglasa, gentlidi;
can chagill, can cadus, can atitin, can comarci
do Dia no do duni.