Trouble Assigning Plot Styles
If you want to stick with CTBs, you will need a CTB file that assigns the printing color Black to all (or most all) of the plot styles (which correspond to drawing colors). For things you want in color, you can either choose a few plot styles whose colors are not being used in the drawing, and assign the colors you want to them -OR- assign a True-Color value to the layer (or object) that you want to plot in color. If you want to use an STB file, you can create a named plot style (called, perhaps, Color) and set that to plot by object color, for layers/objects you want to plot in color (and then assign the desired color to the layers/objects). You would also need at least one plot style that plots Black for the non-color items. It sounds like the drawing you have uses CTB files. The CONVERTPSTYLES command will convert between CTB and STB. Conversion from STB to CTB requires no additional work. Conversion from CTB to STB requires that you have a translation STB file made from your current CTB file, which is done using the CONVERTCTB command.
Accepted solution
A gray plot style column generally means your file is set to use CTB (since you cannot directly set a plot style in a CTB file - you set the color and the associated plot style automatically follows). I have seen files where, immediately after conversion from CTB to STB, the Plot Style column remains locked, and clicking the Refresh button on the Layer Manager palette does not unlock the column. Saving, closing and reopening the file generally restores the column. Do your plot styles read as "ColorXXX", where XXX indicates the color number? If so, you are in a CTB file.
CONVERTCTB does not convert your drawing. What it does is allow you to pick a CTB file that you use and create an STB file that can be used to translate a file to STB. Pick the CTB file that you are using with a file that you want to convert. Create the new STB file using CONVERTCTB. Then use CONVERTPSTYLES, and when prompted to select an STB file, choose the STB file that you created using CONVERTCTB. You should end up with an STB file that has plot styles like OLD_PEN_01, etc.
CONVERTCTB will create a plot style for each unique combination of settings in the selected CTB file (named for the lowest color number where that set is found). When you run CONVERTPSTYLES, the color-based plot styles get replaced with the STB plot style that corresponds to the color-based settings. If you have an office-standard STB file, chances are you will need to reassign the proper plot styles (script files are great for doing this, if you have standardized your layers). Or you could just keep using the conversion STB file, and use the Normal plot style for layers/objects you want to plot in color.
Accepted solution
You should be able to do what you want. If you want the objects on the layer that you want to plot in color to plot with the color that the objects currently have (multiple colors), just set the plot style's Color property to "Use object color", rather than to an explicit color. Assign that plot style to the layer, and keep the plot style of all of your objects on that layer to "by layer". If, on the other hand, you want them all to plot with one color (not necessarily the object color), then you will need to set up a plot style that has the desired color explicitly assigned in the plot style.