The short answer:
One.
Try black or Payne’s grey or sepia.
(If you don’t believe this, then ask yourself how black-and white photography is still a thing so many years after the invention of colour photography.)
The colour theory answer:
Six.
A warm and cool blue, yellow and red.
The typical answer:
The primaries, white, and some earths, perhaps a green.
The Colourist’s answer:
Whatever floats your boat.
My slightly longer answer:
However many you wish, but probably not all in one painting.
Exploring colour is part of the joy of painting, getting to know the personality of each, how it behaves when it’s by itself and how well it plays with others. (I mean: the properties of a pigment and how it mixes with others.) Everyone has their preferences as to how many they want to play with, both at any particular moment and overall.
There is no magic number, it’s about personal preference and it’ll change. If you don’t enjoy colour mixing, you’ll likely use more straight-from-the-tube colour.
How many colours make a successful painting depends on how you use them. Mix them all together and you’ve a muddy mess. Have scattered colour across a composition and you’ve disjointed visual chaos. Worked together, considered and intuitively, it can be intriguing and full of discoveries for a viewer. Never let anyone insist that you limit yourself if you’re enjoying playing with your colours.
When painting with acrylics, I tend to use a subset of colours that has got slightly larger over the past few years (I now need two hands to count the colours, not only one). I do have tubes of many other colours I’ve tried, and occasionally I play with these again. But mostly I use titanium white, Prussian blue, cadmium orange and yellow, lemon yellow, magenta, and?perylene black. Phthalo blue, cobalt blue and phthalo turquoise are also regulars, cerulean blue sometimes joins in. Burnt umber is now neglected as I’ve shifted to mixing greys with orange and blue, and I still don’t like ultramarine blue much (sorry Sharron.
But when it comes to watercolours, since discovering Daniel Smith’s granulating colours, I’ve increasingly been having a ceilidh with colour. Pigments with distinctive characters, not homogenized to all behave the same way like well-socialized, good little colours. I’m not using every single colour in every single painting, and the more successful ones do have fewer colours, but I think these also work because they retain the joy of mucking about with all the colours.
When I moved to Skye 10 years ago, my watercolour set looked like this:
My current watercolour box looks like this:
It feels indulgent to have so many colours, but it’s also so joyous. Sometimes one is all you need, but how will you know which one unless you’ve tried many?
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A big thank you to all my Patreon supporters and subscribers; you enable me to spend more time writing. In the past six months I’ve written twice as many blog posts as I did in the same period last year, as well as create my “Word Prompts” drawing workbook and my “Think Like an Artist” book. Thank you too all my followers, readers, and encouragers. Here’s to whatever the next six months bring!