I want from green to red. Sory if the my english not clear.
In unity very easy: Color.Lerp(Color.Green,Color.Red,1-health*0.01);
You can do it with an effect. Maybe set color? and just set the red, green and blue parameters from events.
Please paste code here because i dont understand.
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When you add an effect you can use an action in the event sheet to do it. My machine doesn't like webgl so I won't make an example.