Design in greyscale, black and white. And it's printed with one ink. It doesn't matter what color that ink is. It's still just one color.
Don't use Pantone or spot colors, just design everything with black and white. The printer will use whatever ink color they choose.
There is never a reason one would need to use one spot color to design something like this. There's absolutely no benefit to designing a one color print project using one spot color. All you essentially do is create more work for yourself. Whether you use greyscale/black & white or a single spot color, in the end all that's output is a single color plate. That single plate is put on a press and the color is chosen by the press operator. Just because you go to all the work of setting up a file using Pantone 286U, the press operator can still simply put Pantone 185C into the ink wells for the press and run the job red rather than blue.
If your client isn't conceptual enough to understand that all you need is one color (black), you should still design the ad in black. Then for approval simply tint or apply a temporary color overlay to shift the color for client approval. I've never had a client fail to understand that the color is determined by the ink on the press and that the design still needs to be black and white.