Brown oil paints are generally earth colors, that is, the pigments in the paint are found in the earth. They come in many shades, and are often overlooked in painting because they are not flamboyant. However, brown paints are extremely useful in a number of ways. Some tones are translucent, and can be used for glazing -- creating thin layers of paint that gently color an area. They mixed well with other, more brilliant colors to create a more cohesive color harmony that is achieved with rich, saturated hues.
Traditionally taken from the earth around Siena, Italy (though it picked up an extra "n" along the way), sienna comes in two varieties, burnt and raw, and is one of the two fundamental earth colors in painting. Raw sienna is lighter, a middle brown color that looks a bit like peanut butter, with a warm tinge. Burnt sienna is darker and rich, chestnut brown.
Umbers (from the Italian word for shadow) are darker than the sienna paints, as their name implies. Umber also comes in a burnt and raw variety. Raw umber is greener and darker, and is a good substitute for black, which can look very extreme in painting. Burnt umber is more of a natural dark brown, and mixes well with yellow to darken it. (Black will often make yellow appear green.) Both the umber pigments are significantly cooler compared to the sienna pigments.
Yellow ocher is a brown mustard hue featuring a translucence ideal for light glazes. It also mixes beautifully with yellows and greens to produce convincing natural shades of foliage and plants for landscape painting. Every painter needs yellow ocher in her box of paints.
Transparent earths quality helps build up glazes, producing jewel-like color. They are more translucent than other earth colors, but also have a hint of color in them, like orange or red, that give them more character than standard brown paints. As they are so translucent, they mix well with white to give clean, clear tints and pastel colors.