Blue-green is the colour our eyes are most sensitive to (roughly 520-530 nm).
Orange stands out more against a green/grey backdrop, but it doesn't attract the eye as much. (Orange would be better out to sea, where the safety clothing needs to contrast against a dark blue-green. Life vests are typically bright orange.)

Also, when the light level is low, our eyes' peak sensitivity moves towards longer wavelengths, but still only yellow-green, so you want the colour to be fairly close to that peak as well.

This is why safety equipment generally has the greenish colour we see today.
Emergency services vehicles are increasingly painted, partly or fully, in that colour.