I havent used the new blendables thingy yet, but in old fashioned way you can use a collection parameter in your PP material to Lerp between scene color and red overlay, then set that PP material in the Blendable slot of your PP volume. Now you should see the transition whenever/wherever you change that scalar collection parameter.
Hello guys, I need to turn my postprocess to make the screen red smoothly. I tried to use a timeline with the values of the blend weight, and if you check with the print string value changes from 0.0 to 1.0. the problem is that apparently the postprocess not refreshed leaving the screen as it was previously. some idea 
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can you show me a practical way to do it? I can not understand how to do ç.ç
You can find an example usage of collection parameters with post process in the project i provided here: [Mini Tutorial] Camera Crossfade - Community & Industry Discussion - Unreal Engine Forums