Blend two textures

I am trying to darken the red part as the players health decreases with a parameter. I am having trouble darkening just the red part of the texture. I have tried multiple blend modes and Lerps. So any ideas how to just darken the red part of the circle, without affecting the other parts?

I sort of figured out the blending, by using a vector parameter instead of a scalar. Now the issue I am having is making it actually change as the player fires
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