I used mixture of plain white and black oil to dry brush SS tankers,
but I read that some uses gray color.
which exact right color to dry brush SS black uniform?

If you're talking highlights, then you want a bit lighter color than what you painted the uniform. You want highlights, yes, but you don't want them to jump out and say, "HEY LOOK WE'RE HIGHLIGHTS!" and sometimes using white makes the effect too strong. You want to suggest depth, not illustrate it like a textbook chart.

BTW Did you paint the uniform full black or did you scale it back to a dark gray? I use the Vallejo Panzer crew uniform black as it is a very dark gray as opposed to pure black - which is just too dark at 1/35 - and then you can use a black wash for the shadows in the clothing and gear.

thank's for pointing that out,yes,highlighting the black uniform.
I usually paint the entire uniform in tamiya flat black.
and highlight it with mixture plain white and black.but sometime white/black look so strong.and I am looking for alternative.
BTW could you provide me the vallejo black gray serial number.

The stuff I am using over here is from their Panzer Aces line and is #333 Germ. Tk Cr. (Black) which is Tank crew black. It is a very dark gray, and when you first open it you might even think it IS black until the light hits it and you see the faintest gray. It's that scale effect that gets mentioned; pure black is just too dark even on a big surface some an airplane kit, and on a figure it just swallows up detail. Plus, how do you show a shadow within pure black? Kind of hard, right? Backing off a bit on the "black" of their uniforms allows you to use the full black down in the creases, the undercuts of their tunics, around insignia and decorations, the small lines in their caps, etc. And then you can use the slightly lighter shade of the gray to highlight the raised ridges of the folds in their trousers, the knuckles of their gloves, the peaks of their caps, etc.

That all being said: Knowing you have at least some Tamiya paint, if you happen to have their NATO Black you could use that as a uniform base. Aircraft modelers use it for tires as rubber just isn't jet black and shiny on WWII planes. I used NATO Black on my panzer wheels until I switched to flat-edge artists' markers instead of paint.

Hope you find that useful.

THANKS!

anyway, for realistic highlights, it's better to use very thinned paint in small layers than drybrushing. It needs a little training but results are much better.