How to partially change the color of a 3D object ?

I want to have 3D Objects with the possibility to change colors of 1 surface of the object. How do I do this ?
We use NX from Siemens PLM Software that supports an *.wrl (VRML 1.0) export

Here the CAD File

We always use AC3D for this. You can load your model into this program and select either an object or an surface and make it black and then export it as vrml 1.0 with the Flexsim settings in AC3D on. When you then load this model in Flexsim, the black areas will show up with the color that you have given the Flexsim object.

I don't know how to do it with the other programs because I don't know what AC3D exactly does so that Flexsim knows what to replace with the color.

Because VRML files are ASCII, you can see what's happening by viewing the files in a text editor. The AC3D "Flexsim mode" export moves the objects that are material 0 (black) above the material definitions in the vrml file. Any objects that are defined above the material definitions in the vrml file will have their Flexsim color showing through. You can do this manually in the vrml files. AC3D just makes the process easier.

In a 3ds file, any objects that have no assigned material and are at the top of the hierarchy will have their Flexsim color showing through. This is a more difficult task for 3ds files and has been made easier in Flexsim5 (the process is described in the "Preparing a 3D File" section of the Flexsim5 Manual).