The phrase "shades of grey" has come to have a particular meaning in our world today because of the salacious novel by EL James, but in this case I am referring to the literal shades of grey that can be mixed with paint!
Respected pastel artist Richard McKinley recommends that pastelists separate out their neutral pastel sticks, storing them apart from the gem-hued intensity of their other pastels, so that the subtle beauty and infinite gradations of the neutrals can be better appreciated and, therefore, used to good effect. Here's a good example of one of his works that illustrates the point:
Indeed, "grey" can be skewed across the entire spectrum, from a rosy-hued pinkish grey through warm yellowish greys through to cool greenish, blue-ish and purplish greys. What they all have in common is the low level of saturation, or intensity. The more pure pigment in a colour, the more "saturated" it is considered to be. Fully saturated colours are a bit like strong spices--a little goes a long way! And if you have one very spicy dish on your menu, it's good to have a number of palate-calming bland foods alongside. Otherwise, your diners will be overwhelmed by the clash of strong flavours, and nothing will be discernible from the cacophony. In the context of McKinley's quiet greyed tones above, the few stronger hues and the small areas of strong value contrast along the left bank of the stream draw our eye, just as he intended.
The other thing greys have in common is a fairly low-to-middle value range. Let's look at McKinley's image again, in greyscale.
Note how little range there is in the dark-to-light scale. Almost all the colours he used are in the mid-to-low value range, with a slightly lighter value in the sky and the small areas of water where he wants higher value contrast against the dark edge of the bank. Again, the effect is quiet and subtle, helping us as viewers to appreciate this gentle scene. It's not a screaming parade---it's a lilting lullaby.
If you are using liquid pigment (watercolour, acrylic, oil) as opposed to pastels, you can mix an infinite range of beautiful greys to suit your needs. Yes, you can, of course, begin by mixing black and white in various proportions. But these greys are often flat and rather dead.
Instead, try mixing complementary colours (violet/yellow; blue/orange; red/green) in various proportions, and then add a small amount of white or black to adjust the value of the resulting grey. As you will see through experimentation, a huge range of neutral greys is thus obtained.
In a recent lesson in acrylic, I used this technique to produce my demo painting. The only paints used in this painting were naples yellow, light blue violet, black and white. As you can see, a full range of warm and cool greys were produced just from these four colours.
While such paintings don't necessarily drag people across a room, they do reward intense and prolonged consideration, making them long-lasting pleasures for discerning collectors.