This is the technical part; buy a colour wheel and trust it.

The rest of the post follows below.

There are a couple of reasons to add colour to an image. One is to make it seem more like the idea or the object on which it is based. A cricket ball is red. Grass is green. Skies are blue. We know all this, and we want to paint accurately…

At this point, the usual disclaimer. Whenever I teach, I am trying to teach you to paint like me. Once you can say why you don’t want to, or why you think something is better, feel free to do that. I did it to my teacher, and sometimes he was right and sometimes I was. There are correct answers in art, but they are very narrowly defined. There is usually a right answer for my particular style or problem; these are the tools I have, and you may use them as long as you like. Disclaimer ends.

The other major reason is to make something stand out. Consider the jarring effect of wearing purple with green, compared to wearing two green items. The clash itself is noticeable, and so is the combination of clothes because of the jarring. This is the basis of the sort of painting I do. When light falls on something green, a faint bit of red in the shadows will make that seem much more solid and realistic. It will also desaturate (take things towards grey) and thus seem duller and more shadowy. If you notice the red, I have failed.

You can make things stand out by putting the ‘hot’ colours at the front, and by having sharper lines. One way to make the lines sharper is to butt things like green and red together. That creates contrast; things closer to us are in greater contrast, so those things pop out more. You can also contrast tone. White and black together are much more noticeable than grey.

So colour does a lot of things. It makes an image better, but it does not make an image. This is one reason I really love oil paints. If something is failing I can walk away from it and have a cup of tea and come back and it is still wet. I do not get that with anything else. If my hand cramps I can rest it, and if I need lunch the painting is still there when I get back to look with fresh eyes. I tend to blend paint on the canvas when working in the large masses of things, so I have need of a medium that will stay wet.

But the big question: how to add colour? There are two things here. If you are looking to paint with transclucent colour (watercolours or glazes of acrylic) then faint underdrawings will work well. You can sketch in blue pencil and let the colour be blotted out, or you can draw precisely and keep to those lines, which is more of a watercolour thing. I would advise tracing all of your work or printing onto paper with faint grey so that you can practice several times on the same thing. I currently have a watercolour design that I have painted four times on different paper as I work on getting it right. Each one looks fine, and each one is different, and the practice is doing me a lot of good.

The other thing: if you are looking to paint with thick-body acrylics, then step away from the paintbrush, and make sure you understand tonal charcoal first. The most important thing of any tonal painting is not the colour, but the black and white design. Look at any acknowledged master painting, and squint to see through your eyelashes, and you will see the black and white version. Photoshop it into greyscale and look closely, and you will still see a good painting. A week or two spent exploring charcoal, even more than tonal pencil, will do you good in the long run. Then you will know which parts are light and which are dark in any painting, and whatever colour mixing theory you use, you will be better set up for it.