A significant part of any painting, whether a pet portrait or abstract picture, is the relationship in colour and tone. Tone is the level of light and shade in the picture, it exists independently of colour and can be calculated on a scale which goes from white through greys to black. The awareness of colour is created by the eye's response to light in different wavelengths; pure colour exists only as light, and as far as painters are concerned, all colours are also modified by tonal value. Pigments are not pure colours, and their range and behaviour is completely different, so painters can only hope to imitate the true colours of nature, by being adept at using their palette and visual tricks. Paintings are not mirrors, but abbreviations of reality, which make sense if key colours, tones and outlines have an internal logic and consistency.
Since colour and tone are both dependant on light, it is a great help if both the lighting the subject, and the environment in which the painter works, is kept under control. Portrait painters usually prefer to light their subjects from only one direction, enabling them to become familiar and easily predict the way the light falls on a face. Others will choose to paint at a particular time of day when the lighting conditions are an average quality, and shadow lengths are suited the painters style. An additional technique uses diffused light, which softens light and removes harsh shadows. To accomplish this, paper saturated with drying oil and allowed to dry can be used to cover windows, or pasted onto screens to create the desired result. If you use pale coloured paper it also has the benefit of harmonizing the colours in the subject. This is also a useful trick if you are having to paint in a room which is subject to full sun. Alternatively, studios that are too dark should be painted white to reflect as much light as possible back into the room.
Using a Claude glass
A Claude glass is a black-coated mirror used to assess the tonal relationships in a picture. Being black, it nullifies the colours in the subject reducing them to tonal values. This then makes it simple to see the tonal relationships in the picture with any errors quickly being shown. You can make your own Claude glass by painting one side of a piece of glass with black paint or by placing a piece of smoked glass in front of a mirror. For landscape paintings, it often better to use a Claude glass with a curved surface, this enables you to regard the scene in its entirety.
Above all, it is important that you know and understand the colours you use on your palette. This will only come with experience, but to be a great colourist an artist has to know how colours relate to each other, how each individual colour reacts when blended and how well it performs as tints. Skillful colourists exploit favored colour effects, tending to use a limited palette of colours but knowing and understanding them perfectly.