Having some neutral values at hand is very handy. You can use them for a value scale or to mix with higher chroma colors (of the same value) to make them more neutral. If you have problems judging the value of a high chroma color squint your eyes.
While a commercial Munsell value scale can be quite pricy you can try to substitute it with free color swatches from the home improvement store. Compare them carefully – some will look somewhat greenish, some reddish ect. Coose the most neutral looking ones.
I’ve got the Munsell student book, but the problem with the matte chips from the book is, that you can not place paint dabs onto it and whipe it off afterwards. So what you need is some kind of glossy color swatch – and those from the home improvement store should be glossy.
As you can see in the above picture I wasn’t able to match some of the neutrals perfectly due to the matte chips (and little patience). Some are spot on but some are too warm. So I borrowed the neutrals from someone who has the big Munsell book, which contains chips with a glossy surface. Now I was able to check the neutrals better.
The above picture shows the paints that I’ve used. But I had a hard time mixing some neutrals, so I needed a little bit of either Cadmium Yellow or Cadmium Red – depending on the direction I needed to go. Note that mixing Titanium White with Ivory Black will result in a cool color and not a neutral as one would expect. Ivory Black actually is a very dark blue (try it yourself by mixing it with yellow – you’ll get a green).
In case you go to the home improvement store and look for neutral swatches, compare all of them with each other. In this picture you can see the little differences (from left to right: neutral, neutral, too yellow, too green, too purple, too blue)