Latin caesius 'light blue' (updated)

According to my amateur colleague Marco Moretti, Latin caesius 'light blue (also said of eye colour)' is a loanword from Etruscan caisie-, ceisi(e)-. These forms, together with caizna, ceizna, have exact counterparts in Baltic: Lithuanian gaĩsa-s 'glow, redness in the sky', Latvian gàiss 'air, wheather', gàišs 'bright, clear', gaisma 'light', etc.¹ In my opinion, these words are indicative of prehistoric contacts between Tyrrhenian (the language family to which Etruscan belongs) and a linguistic continuum from which emerged Baltic and Thracian². 

Also Lithuanian gaidrùs 'bright, clear (said of wheather)' and Greek phaidrós 'shining, bright, cheerful' belong here, and Duridanov reconstructs Thracian *gaidrus on the basis of the personal name Gaidre(a)s³. For these forms, an IE protoform *g^wai- 'light, bright'⁴ can be reconstructed, with regular delabialization of the labiovelar in "satem" languages. In my opinion, this is a loanword from Caucasian *ʔVqq^wo-ji- 'white, light, blueish', in turn a derivative of *=eqqwA 'yellow'.

From the same Caucasian source it also comes a different IE protoform *koi-t^ʔ-, found in Lithuanian skaidrùs 'clear, bright', skáistas, skaistùs 'bright', Germanic *xaid-u- > Gothic haidus 'way, manner', *xaid-a- > Old Norse heiδ 'clear (said of weather)', heiδr 'clear, cheerful', *xaid-ra- > Old High German heitar, German heiter 'cheerful', and to which some IE-ists link the Latin word⁵.

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¹ As Etruscan didn't possess voiced stops, c /k/ would be the rendering of a former /g/.

² This continuum was broken by the irruption of people from the Pontic Steppes, whose languages interacted with the autochthonous ones and gave raise to Dacian (a close relative of Indo-Iranian) as well as Greek and its nearest relatives (Macedonian, Phrygian, Armenian).

³ I. Duridanov (1969): Die Trakisch- und Dakisch-Baltischen Sprachbeziehungen, p. 75.
⁴ Ortodox reconstructions are *g^whai- and *g^wheH2i-. See R.S.P Beekes (2010): Etymological Dictionary of Greek, p. 1544.
⁵ M. de Vaan (2008): Etymological Dictionary of Latin and the Other Italic Languages, p. 82.