Create uniform variable in your fragment shader (I assume You are doing texture stuff there), and change it’s value every frame.
In the shader do something like this:
vec4 lookup = texture2D(...) <- your texture
lookup.r *=, +=, -= ...etc. yourUniformVariable;
(the same for other components )
I don’t know if this is the best way to do that, but it works.
ps. sorry for my bad English.