I was looking at one of the answer you gave here : https://godotengine.org/qa/9084/there-method-use-canvas-drawing-top-the-buffer-the-previous
I messed around with your code and maybe found an answer to your current question.
var vpgrtt = viewport.get_render_target_texture()
board = get_node("Control/Sprite")
board.set_pos(viewport.get_rect().size / 2.0)
(refer to your answer in the linked thread for more details about the code)
since you added a sprite to the control node, you can play around with the modulate option!
Note that the sprite can be empty (it is in my case) and serves only as reference and modulation.
It may be a solution, tell me what you think.