Plane mesh not accepting color changes from editor

Hi there,

I'm having troubles working for the first time with materials. I've created a Plane Mesh, and both in the scene and in the game, it is rendered with a purple colour. I want to change this, so I create a new Material with a Standar Shader (blue, for instance) and assign this material to the mesh. Now, the plane mesh becomes rendered with the black color.

Can you help me identify the problem? I don't know if I'm missing something that must be done in the material besides assigning the color, or if maybe it has to do with illumination.