Are you working on the Model tab, or on a Layout tab? A layout tab can be set to display plot styles. If your plot styles assign a specific color (rather than Object Color), like Color 7 "White/Black" and you have a dark background, then all the objects will display in that color, no matter what color is assigned in AutoCAD.
For example, I set up a CTB-based file that has the monochrome.ctb set as the plot style on a layout, and I changed the Sheet / layout:Uniform background color from white to 33,40,48, which is a very dark gray. I drew a number of objects (in model space through a viewport, although they could have been in paper space as well) on four layers, called Layer 1, Layer 2, Layer 3 and Layer 4. The color of all objects is set to ByLayer, and the layer colors are red, yellow, green and cyan, respectively. Here is a screen capture of the layout, without having it set to display plot styles:

If I now run the PAGESETUP command, and choose to modify the currently active page setup, I can enable the display of plot styles by checking the Display plot styles toggle at the upper right of the Page Setup dialog. Note that the monochrome.ctb file is set current, which maps every plot style to the color Black.

Clicking OK twice to accept the changes, dismiss the dialogs and return to the drawing gives me this, with everything in white:

Perhaps that is how your file is set up?
I will occasionally do this, with a white background, to have the layout show in a sort of permanent "plot preview" mode.* Non-plot layers still show - but we have a special plot style for those that changes them to Cyan, which, on a white background, can still be seen but with far less impact than the all-black linework that should appear for items that will plot (for contract documents). This is also a good test to find objects with incorrect plot style settings.
* - Depending upon how graphically dense your layout is, and how powerful your computer is, you may find that leaving layouts with plot styles displayed results in slow layout generation times. After checking the graphics, I usually toggle the display of plot styles back off.
David Koch
AutoCAD Architecture and Revit User
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