Can anyone explain to me why hard-metal endoskeleton Terminator models always have red-colored eyes? Is this for some technical reason?
They aren't always red
It's worth noting that at the end of season 2, we see that her endoskeleton has classic Terminator red eyes. The show's writers claim that her (hardware) eyes were always red and that the blue glow was the result of some form of software control of the filters used in her eyes.
Interestingly, the choice of eye colour for both models seems to have been motivated (out of universe) to distinguish these anti-terminator-terminators from their classically eye-coloured brethren