If you want to use AutoCAD to plot Inventor drawings as monochrome printouts, you need to watch the proper plot style.
DWG drawings saved from Inventor (Save copy as) use object colors assigned using RGB components (truecolor, an universal method). But the standard method for black-and-white print from AutoCAD is based on color mappings assigned from a CTB plot style file Monochrome.ctb. This file maps the monochrome print "colors" to specific indexed colors from the standard 256 color table (ACI) - it cannot be used for truecolors defined by RGB components (it would be neccessary to define 16.7 milions individual mappings).
The solution is simple - you can just switch the drawing to "named" plot styles, STB. This can be done with the CONVERTPSTYLES command. During this command, you specify the STB file mapping the monochrome output to named styles.
You can convert your existing Monochrome.ctb file to STB with CONVERTCTB. STB files are stored in the folder "Plot Styles" - e.g.:
C:\Users\MYNAME\AppData\Roaming\Autodesk\AutoCAD 2012\R18.2\enu\Plotters\Plot Styles
You can also automate the conversion for automatic execution or to a menu button - use a simple macro: (command "_CONVERTPSTYLE" "MyMonochrome.stb")
Another, maybe simple method is to just switch the printer to a monochromatic mode (if the driver supports it). In the PLOT dialog click the Properties button and on the Device settings tab choose the Custom properties button. The given driver can then offer functions like "Print in grayscale" or "Black and white" - e.g. the Color tab for HP plotters.