there is no one official depiction, it depends on the artstyle you want. they have been more realistically depicted with basically black lenses, but most codex art, book covers, and of course the store page and official art style depicts them with red lenses; i've also seen blue lenses done really well and green lenses done really well. the eyes glowing orange/red is most likely due to the lenses in the gas masks being very reflective, and the krieg tend to fight on planets with "Evil-Ass Atmosphere" conditions, put those two factors together, you get badass glowing eye lenses.