On my Pixel using GrapheneOS, I can go into Accessibility settings and enable gesture support for toggling Grayscale support, without the need for Tasker. I'm unsure how other Android ROMs stack up but I imagine stock Pixel has this setting.
It's a great feature, just swipe up from the bottom with two fingers whenever I need color.
I'm _almost_ certain that comes with any Android ROM from 9.0 up as a standard accessibility setting. All my Samsung devices have had it for a few years.
It's a great feature, just swipe up from the bottom with two fingers whenever I need color.