In this case we will use the 3D model that comes by default in clip studio pain pro. But before I start I want to say that you can find all kinds of 3D models in clip studio assets. For example, if you want to make a more mechanical robot maybe it would be better to use a skeletal 3D model. Or if you want to make a robot that seems to be covered with armor then I would recommend a muscular 3D model. But in any case everything will depend on which one you find most comfortable.
First, we will drag our 3D model to the work area and choose a pose. then, we will deactivate> Use texture.
Now in the 3D model layer we will open the gradient window, which by default the gradient bar will be black and white.
Moving the triangles that are below the gradient bar we will adjust to the 3D model until it assimilates the metal. Then, at the end we will accept and to the gradient layer that is created we merge it with the 3D model layer. By right clicking on the layer, with the option> Merge with lower layer.
Something that we have to take into account when making a robot are the joints. These parts are essential for the perfect movement of our robot. For this simple robot illustration I will cut them with the tool> Eraser> Soft.
In this part you will see how I rejoined the joints .. If it is fixed there is not much trick. I only joined them with the tool> brush> G, and also on the edges I had cut, the brush passed a little.
I also made some extra cuts and then passed with the tool> pen> G.
You can apply gradient again with the window> Gradient Map and get other colors.