Assuming "Zenithal" highlighting means using different darkness primer beneath your main colours, you could do a few things without an airbrush.
First, if you start with white primer, a coat or two of mid-grey wash, followed by a dark grey / black wash (only in the darkest areas) could work.
If you start with grey primer, you could wash it black, then drybrush white. Probably the easiest option.
If you start with black primer, you could go with a heavy drybrush of grey, followed by a light drybrush of white.
You could also go white primer with a black wash. Fastest option, but probably the least smooth for colour transitions.
After that, you just use very thin paint. Thin enough that the white upper edges of the details show through as highlights.
I normally do this over white primer, then add a darker wash afterwards. It's fast, and you get nearly the same effect of "whitened" highlights, mid-tone, and darker recesses. I find that to be faster, overall, than any of the first three, and I can then drybrush / highlight if I so desire afterwards.
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