Sift the icing sugar and matcha powder into a small bowl. Add in a few drops of lemon juice or plant-based milk at a time until it reaches the desired consistency. You want a thick icing that drips off a spoon in a slow steady drizzle. Add more matcha powder for a more intense color (and flavor).
Icing colored with juice
Sift the icing sugar into a small bowl. Add in a few drops of juice at a time until it reaches the desired consistency.
Icing colored with berries
If using frozen berries, thaw them on your kitchen counter or in the microwave. Mash them with a fork and run the puree through a fine-mesh strainer. Slowly add the strained purée to a small bowl with icing sugar. If the mixture is too thick, add a few drops of lemon juice or plant-based milk until it reaches the desired consistency.