First add a PostProcessComponent to your character.
Then
What we are doing here?
So, we add a postprocesscomponent to my character.
Then on The Begin Play event we create a new Dynamic Material Instance and we store it in the variable MYPPMAT (My PostProcess Material).
After that we set the component to unbound and then we assign the new material instance to postprocesscomponent settings.
On the key event P and M we get a reference to material and we read the scalar parameter name value of “Gamma” and we add/subtract 1, after that we set the new value.
Surely exists some better solution. You are free to find it.