The term watercolour refers to paints that use water soluble, complex carbohydrates as a binder. Originally (16th to 18th centuries) watercolour binders were sugars and/or hide glues, but since the 19th century the preferred binder is natural gum arabic, with glycerin and/or honey as additives to improve plasticity and dissolvability of the binder, and with other chemicals added to improve product shelf life.
Bodycolor refers to paint that is opaque rather than transparent, usually opaque watercolour, which is also known as gouache. Modern acrylic paints are based on a completely different chemistry that uses water soluble acrylic resin as a binder.
watercolour painters before c.1800 had to make paints themselves using pigments purchased from an apothecary or specialized "colourman"; the earliest commercial paints were small, resinous blocks that had to be wetted and laboriously "rubbed out" in water. William Reeves (1739–1803) set up in business as a colorman about 1766. In 1781 he and his brother, Thomas Reeves, were awarded the Silver Palette of the Society of Arts, for the invention of the moist watercolour paint-cake, a time-saving convenience the introduction of which coincides with the "golden age" of English watercolour painting.
Modern commercial watercolour paints are available in two forms: tubes or pans. The majority of paints sold are in collapsible metal tubes in standard sizes (typically 7.5, 15 or 37 ml.), and are formulated to a consistency similar to toothpaste. Pan paints (actually, small dried cakes or bars of paint in an open plastic container) are usually sold in two sizes, full pans (approximately 3 cc of paint) and half pans (favored for compact paint boxes). Pans are historically older but commonly perceived as less convenient; they are most often used in portable metal paint boxes, also introduced in the mid 19th century, and are preferred by landscape or naturalist painters.
Among the most widely used brands of commercial watercolours today are Daler Rowney, Daniel Smith, DaVinci, Holbein, Maimeri, M. Graham. Reeves, Schmincke, Sennelier, Talens, and Winsor & Newton.
Thanks to modern industrial organic chemistry, the variety, saturation (brilliance) and permanence of artists' colors available today is greater than ever before. However, the art materials industry is far too small to exert any market leverage on global dye or pigment manufacture. With rare exceptions, all modern watercolour paints utilize pigments that were manufactured for use in printing inks, automotive and architectural paints, wood stains, concrete, ceramics and plastics colorants, consumer packaging, foods, medicines, textiles and cosmetics. Paint manufacturers buy very small supplies of these pigments, mill (mechanically mix) them with the vehicle, solvent and additives, and package them.
Many artists are confused or misled by labeling practices common in the art materials industry. The marketing name for a paint, such as "indian yellow" or "emerald green", is often only a poetic color evocation or proprietary moniker; there is no legal requirement that it describe the pigment that gives the paint its color.
To remedy this confusion, in 1990 the art materials industry voluntarily began listing pigment ingredients on the paint packaging, using the common pigment name (such as "cobalt blue" or "cadmium red"), and/or a standard pigment identification code, the generic color index name (PB28 for cobalt blue, PR108 for cadmium red) assigned by the Society of Dyers and Colourists (UK) and the American Association of Textile Chemists and Colorists (USA) and known as the Colour Index International. This allows artists to choose paints according to their pigment ingredients, rather than the poetic labels assigned to them by marketers. Paint pigments and formulations vary across manufacturers, and watercolour paints with the same color name (e.g., "sap green") from different manufacturers can be formulated with completely different ingredients.
"Transparent" colors do not contain titanium dioxide (white) or most of the earth pigments (sienna, umber, etc.) which are very opaque. The 19th-century claim that "transparent" watercolours gain "luminosity" because they function like a pane of stained glass laid on paper – the color intensified because the light passes through the pigment, reflects from the paper, and passes a second time through the pigment on its way to the viewer—is false: watercolour paints do not form a cohesive paint layer, as do acrylic or oil paints, but simply scatter pigment particles randomly across the paper surface. watercolours appear more vivid than acrylics or oils because the pigments are laid down in a more pure form with fewer fillers (such as kaolin) obscuring the pigment colors. Multiple layers of watercolour do achieve a very luminous effect because of less fillers obscuring the pigment particles.
Staining is a characteristic assigned to watercolour paints: a staining paint is difficult to remove or lift from the painting support after it has been applied or dried. Less staining colors can be lightened or removed almost entirely when wet, or when rewetted and then "lifted" by stroking gently with a clean, wet brush and then blotted up with a paper towel. In fact, the staining characteristics of a paint depend in large part on the composition of the support (paper) itself, and on the particle size of the pigment. Staining is increased if the paint manufacturer uses a dispersant to reduce the paint milling (mixture) time, because the dispersant acts to drive pigment particles into crevices in the paper pulp, dulling the finished color. Granulation refers to the appearance of separate, visible pigment particles in the finished color, produced when the paint is substantially diluted with water and applied with a juicy brush stroke; pigments notable for their watercolour granulation include viridian (PG18), cerulean blue (PG35), cobalt violet (PV14) and some iron oxide pigments (PBr7). Flocculation refers to a peculiar clumping typical of ultramarine pigments (PB29 or PV15). Both effects display the subtle effects of water as the paint dries, are unique to watercolours, and are deemed attractive by accomplished watercolour painters. This contrasts with the trend in commercial paints to suppress pigment textures in favor of homogeneous, flat color. Grades Commercial watercolour paints come in three grades: "Artist" (or "Professional"), "Student", and "Scholastic". Artist watercolours contain a full pigment load, suspended in a binder, generally natural gum arabic. Artist quality paints are usually formulated with fewer fillers (kaolin or chalk) which results in richer color and vibrant mixes. Conventional watercolours are sold in moist form, in a tube, and are thinned and mixed on a dish or palette. Use them on paper and other absorbent surfaces that have been primed to accept water-based paint. Student grade paints have less pigment, and often are formulated using two or more less expensive pigments. Student watercolours have working characteristics similar to professional watercolours, but with lower concentrations of pigment, less expensive formulas, and a smaller range of colors. More expensive pigments are generally replicated by hues. Colors are designed to be mixed, although color strength is lower. Hues may not have the same mixing characteristics as regular full-strength colors. Scholastic watercolours come in pans rather than tubes, and contain inexpensive pigments and dyes suspended in a synthetic binder. Washable formulations feature colors that are chosen to be non-staining, easily washable, suitable for use even by young children with proper supervision. They are an excellent choice for teaching beginning artists the properties of color and the techniques of painting.
As there is no transparent white watercolour, the white parts of a watercolour painting are most often areas of the paper "reserved" (left unpainted) and allowed to be seen in the finished work. To preserve these white areas, many painters use a variety of resists, including masking tape, clear wax or a liquid latex, that are applied to the paper to protect it from paint, then pulled away to reveal the white paper. Resist painting can also be an effective technique for beginning watercolour artists.
The painter can use wax crayons or oil pastels prior to painting the paper. The wax or oil mediums repel, or resist the watercolour paint. White paint (titanium dioxide PW6 or zinc oxide PW4) is best used to insert highlights or white accents into a painting. If mixed with other pigments, white paints may cause them to fade or change hue under light exposure. White paint (gouache) mixed with a "transparent" watercolour paint will cause the transparency to disappear and the paint to look much duller. White paint will always appear dull and chalky next to the white of the paper; however this can be used for some effects.
Courtesy of Wikipedia