'Watercolour is a unique medium as it relies on a general process of working light to dark,' says Paul Talbot-Greaves. 'It takes quite some practice to visually unpick a scene, looking for the lighter values and colours to lay down first – it really is a backwards process. 

'Although this picture of sweet peas looks at a purist approach using negative painting to form the lights, there are cheats and other ways of achieving light on dark to explore in watercolour. Light can be placed on dark if gouache is used, although this is generally best reserved for the end of the painting.

'Masking fluid and masking tape can also be used to reserve lighter passages or white paper, allowing you to freely apply a background without worrying about painting around the positive shapes. The only downside to this is that the masked shapes remain hard-edged and may not be in the right place at the end if you change the picture slightly.

'I prefer to work in a traditional sense to begin with, establishing the main building blocks of the painting with values and shapes first, using as much negative painting as I can, but ignoring small or difficult light-on-dark details. These are left until last, where I use gouache to paint them in place, light on top of dark.

'If you are not familiar with gouache, it is best described as opaque watercolour. Use it thickly from the tube for highlights and dense colour or dilute with a little water to make it semi-opaque for thinner veils and light textures. Using lots of gouache early in the painting can get messy if it begins to mix into your regular colours, so I use only a small amount in a limited way. One positive attribute is that it is easy to remove if your efforts don’t go to plan. Whichever way you work with watercolour though, you are not going to avoid facing the kingpin of the medium: that of negative painting'.

Negative painting

Negative painting is the art of placing darker shapes to make the lighter shapes stand out.

Think of a section of chicken wire, light against dark. To paint it in a traditional watercolour manner, you would apply darker value to all the hexagon shapes, which will create an overall illusion of chicken wire against a dark background.

Negative shapes such as these aren’t always obvious at first glance and, especially for the beginner, it can be a hard concept to grasp.

Negative painting isn’t unique to watercolour, but the general light-to-dark application means it is probably used most widely when watercolour painting. Learning to see both positive and negative shapes is the crux of seeing the whole image in any medium.

In this instance of painting sweet peas, I began with the lighter values using thin washes of colour to allow lots of white paper to show through. 

In an oil painting, it would be quite the opposite and the darks would be established first.

Next, I worked my way through the mid-value range, cutting around the lighter shapes to make them stand out. This process can also be followed in opaque media such as  oil, but with opacity there is also the ability to overlay lights on top of darks, making the process a little more versatile and easier to figure out.

To practise negative painting in watercolour, find a simple subject involving light on dark and try to describe those lights by applying only dark values on watercolour paper. Work your way through more exercises and increase the difficulty level each time. Many layers of negative painting can give a great depth, of which foliage makes a complex and challenging example.

Counter change

To pick out the negative and positive shapes in a subject, squint your eyes and look for the values, particularly contrasts. These are the building blocks of any painting.

Often when looking at an image in terms of value you will notice changes from dark to light and light to dark. When multiple shapes are involved, as here, these shifts in contrast occur constantly.

An example of counter change is where a shape, such as a leaf, is seen light against dark at one edge and dark against light at the other. Counter change creates interest and illusion on the eye and helps to make the painting engaging.

Colours

For this painting I used complementary colours – green and red-violet. 

Whilst it can be tempting to lay one colour over the other, the transparent nature of watercolour will lead to them optically mixing on the paper, resulting in grey.

To maintain fresh colours, it is imperative to paint them in place and only allow mixing where they meet. You can see how I did this in the background flowers to create soft edges.

In circumstances such as this, I use two brushes, one containing green and the other magenta to avoid cross-contamination and also for quicker application.

For harder edges, the process is a little easier as drying between colours naturally keeps them apart. For the bright flowers I chose to use intense staining colours.

Quinacridone magenta was a good match for the sweet peas and I introduced Winsor violet to darken it rather than adding a neutral such as black or a complementary colour. This helped to reflect the light intensity within the petals rather than dull it.

To make the greens I mixed Winsor blue (red shade) with raw sienna to create a slightly dull green and by alternating the mix I was able to shift between cool and warm hues. I brightened the green by replacing the raw sienna in the mix with new gamboge in places.

Demonstration: Sweet Peas