The Base-Part is a Random color.

If the Base-Part is a not Green:

Forces toolbar > Display Forces = Show Force Vectors.

When this icon is enabled to display Force Vectors, each Part is given a random color.

To make the Part Green, you should:

Click the Forces toolbar >Display Forces = Hide Force Vectors

When the icon is disabled the Base-Part should be Green.

The Base-Part is hidden by a Plane.

This is because:

Visibility toolbar > Show Solids in Mechanisms is enabled

When we show the Solid elements in Mechanism-Editors, we also show the Planes.

To hide the Planes:

Click Visibility toolbar > Show Solids in Mechanisms disabled.

- or -

Click Display toolbar > Display Planes to Hide Planes

Visibility toolbar > Show other Kinematic and Sketch elements

If this Visibility Toggle is active, and if another Mechanism-Editor is in front of the active Mechanism-Editor, you will see the Pink* Other Base-Part (* Default Color)

If you spin the model you will be able to see the Base-Part of the active Mechanism-Editor.

See Application-Settings > Graphics tab > Display Colors > Background Sketch