Hi Michael,

Same thing happens here at work on NX6, exact same error.
I did some quick tests to see if there was any difference with different settings and export options using the file attached.
The sequence was;

*save as step with colours in the model, Write .Mtl file, checked.
*save as step with no colours (default colour) in the model, Write .Mtl file, checked.
*save as step with colours in the model, Write .Mtl file, unchecked.
*exported. as step with colours in the model, Write .Mtl file, checked.
*exported. as step with colours in the model, Write .Mtl file, unchecked..
*save as step with colours and object name in the model, Write .Mtl file, checked.
*save as step with colours and object name in the model, Write .Mtl file, unchecked.

All have the same error result with NX6.
I also remembered explaining how NX handles it's layer system, where colours are handled separate and not part of the layer.
http://moi3d.com/forum/index.php?webtag=MOI&msg=2581.38

If you need any other info, let us know.

Cheers