i want to add some tiny spots on the face of a 3d character.

small spots of any colour on random positions on the face of a 3D character.

You probably want to do this with Shaders. You can find a quick introduction over here:
Unity Shader - introduction
However, shaders, if you’ve never worked with them before, aren’t a topic to grasp on one afternoon.

Another solution might be to use an “animated texture”. Though I can’t tell, if you’d still be able to randomize those spots then without further coding.