To produce Zn nanoparticles You should observe pH value. When it grows above 6, You will gen Zn(OH)2. Add some HCl, work under fume hut.

Thanks Tomasz! I've already tried to acidify the solution but the reaction didn't work! I also tried working on organic solvent but the result is the same...

Here I found solubility of ZnO as a function of pH. Check if Your product behaves the same. You can also try to reduce Zn as anion, ZnO dissolved in ammonia or NaOH. In alcalic solution it should be better. Which sodium borohydrate do You use, NaBH4 or NaBH3CN ?

Zn nanoparticles doesn't have to be grey, NP colour depends on the size distribution and surface properties.

I have been working with Ni, Co and Cu nanoparticles, one of the biggest problems is to obtain metallic nanoparticles because they are very reactive, however using approtic solvents as THF and surfactant as tetrabutylammonium boronhydride, I could obtain good results, of course you should mantain inert atmosphere during reaction (N2 or Ar). good look!!

Per one report (http://www.ijsciences.com/pub/pdf/V1-201212-16.PDF) zinc nanoparticles are "thermodynamically unstable".  You could try the reported protocol and use the resulting suspension immediately.  If the particles are small enough to exist as a colloidal suspension, any color is possible.