If you want to follow the Munsell colour system, you can mix your own neutral greys according to the grey scale you can find on various websites, including the excellent and very informative Golden Paints website (I will post a link). Golden now makes Williamsburg paints, by the way. Alternatively, you can buy these pre-mixed neutral greys from Williamsburg. They come in a range of only 4 paints (NG 2, 4, 6 and 8) so you have to mix the intervening greys yourself. Eg you would mix grey no 3 by combining greys no. 2 and 4 till you get the appropriate value. Just to make things difficult (and isn’t that what we love about art?) it won’t necessarily be a half and half ratio; instead, you need to judge it by eye, using an accurate print-out of the Munsell grey scale as a reference. It’s not that hard if you’re not too pernickity.
You can buy these Williamsburg greys from Parkers Art store in Sydney or order them through Adart in Hobart. Seniors art supplies in Melbourne probably has them too. I have found Parkers and Seniors art supplies very quick in sending materials. Adart are good too but they rely on getting supplies like these from Langridge in Melbourne and they seem to be very slow in sending stuff to Hobart.
If you do try to mix them yourself, be aware that black plus white do NOT create a neutral grey. This is because ivory black is really an extremely dark blue (not that you can see this with the naked eye but it becomes more apparent when you mix it with white). And not only that, but titanium white is quite a bluish white. You can use lead white which is a warmer white (and in my opinion, nicer in every way) but even that won’t get you entirely out of trouble.
If you create a grey “scale” or set of values (aka tonal) gradations with just black and white, you will end up with a very cool set of tones, which will not work for desaturating colours in the way the Munsell system intends. Instead they can distort the hue (or actual colour) of the paint you’re trying to desaturate because they’re not neutral (neutral meaning neither cold nor warm). Eg if you try to darken or desaturate a lemon yellow (if you’re painting a lemon, say, or the hideous crimplene slacks of an elderly American tourist) using a grey made of simply black and white will tun your lemon yellow green. Not a good look, unless that’s what you’re looking for.
Graydon Parrish, a great proponent of Munsell’s system, has some good recommendations for mixing your own neutral greys. I’ll see if I can post a link to his internet pages. Basically what is needed is the judicious addition of varying amounts of burnt sienna or burnt umber to counteract the coldness of the black and white paints. Also check out “Classical Atelier at home” which I’m also about to post a link to.