Does greyscale/grayscale mode save battery life?

If the screen is LCD or LED then, there is no battery saving as answered on this question Will grayscale increase or decrease battery consumption? on AskDifferent.

However with an AMOLED screen, there is an advantage as the black pixels will be turned off.
According to the article Why You Should Use Always Use Dark Mode on Phones

That said, the differences in power Google discovered between using an app normally and using its "night mode" (or dark mode) were striking, particularly for OLED-based phones. For example, when testing an AMOLED-based Google Pixel, Google found that Google Maps uses 63 percent less power when it Night Mode compared to normal mode. By contrast, an LCD-based iPhone 7 saw no change in power consumption

Although Night mode is not grayscale, the advantage of switching off the dark pixel should help in saving the battery.