How to add screen filter with tasker?
Is this what you're looking for?

(Click image to enlarge)



I'm not good in drawing so excuse my skills for not providing a breathtaking scene at first glance. However, with multiple attempts, you may come close to exactly what you need.

The low-light section in the screenshot is caused by an overlay shown by Tasker. Tasker can do it using Scenes.

Go to Scenes tab and start making your scene.


Choose the size of your scene. For screen filter, cover the whole screen.
Add Rectangle as the element and encompass the whole scene with it.
Tap Rectangle and choose a custom color which would be acting as the screen filter.
Create a task and action: Scene &rarr; Show Scene 


Name: select your scene
Display As: Overlay
uncheck Show Exit Button
make other changes if you need any



Execute the task and there you go.

Note: The overlay in my screenshot deliberately doesn't cover the entire screen.