Sadly no, it is impossible to do it by script, you need to do it with a shader that lerp between two textures.
The closest thing to do that would be to modify the texture itself at runtime, and that's frankly very complicated, especially with playmaker (I am currently trying to create some texture modification actions, but due to fact that you need to use standard arrays,it is really a pain).
I advise you to take a look at Amplify Shader in order to have a visual scripting interface (like playmaker) for the shaders, that's what I use personally, and it's really good for that IMO.