Different Colored Model

Why are all the parts that I join onto a model colored differently when in sculpt mode?

Are any functionalities affected? How do I make it all one color?

Have you joined them with ctrl J in object mode so it is 1 mesh?

Check your viewport shading settings

If you have the above selected and they are different objects, then each object will have a different colour

Those are “Face Sets”. They are a way to mask different parts of the mesh, which you can define in various ways. When you join a new part it automatically assigned its own face set.

If you just want to make them invisible: disable the overlay in Sculpt Mode in Viewport Overlays > Face Sets.