After adding a Matter light to Tasker you can select it with the helper (magnifying glass). You'll be able to either select its name or ID. If you have multiple devices with similar names you might prefer to use its ID to make sure you're controlling the correct one.
Set
Select if you want to turn on, off or toggle the light. You can use the magnifying glass helper to select the correct option. Possible options are "On", "Off" or "Toggle".
Colour
A color in the #RRGGBB format to use for the light. If the light is turning off, this value will be ignored. If Alpha is used (with the #AARRGGBB format), Alpha will be ignored. Again, use the magnifying glass to help you out.
Brightness
A value from 0 to 100 that controls how bright the light will be if turned on. If turned off, this value will be ignored.