Further to the comments, the problem is because you changed the background colour in the settings: Simulate Colored Paper. If you do that, and also have the Overprint Preview enabled, this will cause the problem. You can toggle the Overprint Preview on/off by hitting Alt+Shift+Ctrl+Y
Note that the option to simulate paper is for showing what will happen if you print on coloured paper. So, I'd suggest you don't use that option unless you specifically want to do that. Also if you print using this option on white paper, the background colour will not print! It's not a real background colour - it's only an on-screen simulation.
TLDR: Don't use this option if you want the background colour to be printed, or to be an actual part of the graphic.
Instead, create a filled rectangle as the background, and lower it to the bottom of the layer stack under the artwork layers.