This might help, but I don’t know if it’s your case: Constant Material Expressions in Unreal Engine | Unreal Engine Documentation
I’ve created a material instance based on a base material whose color is parametrized to set at runtime. I then applied it to a skeletal mesh. I setted up a BP that give a random color when application starts and color is correctly applied at a random value as expected. If I duplicate several times the skeletal mesh, so with same instanced material shared, the color remain the same for every actor: How can I assign a different random color to every mesh?
Thanks for your help mate, but this only works with static meshes unfortunately 
Do you maybe know a similar method compatible with skeletal meshes? thx again
I cant fully understand what you are trying to do but what it sound like is this:
You want have the skeletal mesh’s material choose a random colour every time you put one in the scene.
if this is true then you want to assign the material’s colour inside of the construction script. another way to do it would be to do it in the begin play event but that means the colour’s would only be assigned when the game starts not when you are editing the level in the editor
Thanks for suggestions, I really appreciate your help. Finally I found this way to solve the problem: I converted the skeletal mesh in a Blueprint, and in the graphs I created the “actions” to select random color (and morph targets). The random color value created is then applied to dynamic material instance parameter named “coloreMaglietta”. The following image shows actor graphs, I hope they can help someone else 
