There are two ways you can do this in Carrara,.
You can use the traditional method of selecting a group of polygons and designating that selection as a Shading domain, and then creating a Shader, and applying it to that named shading domain.
You can use a shader function in Carrara, to apply a texture or logo onto a “Layer”, directly onto the model in the shader room.
At the top of the shader room, there are a couple of shape tools, (rectangle polygon and oval) as well as a Pointer tool to manipulate the shape layer once you create it.
You simply draw a “layer” shape, onto your model in the shader preview window.
this creates a new shader “Layer” which allows you to add another texture or colour to that layer shape.
Hope it helps