Previous painting media, fresco, tempera, and mosaic were all opaque and flat.
Linseed oil is a transparent binder that dries naturally and can be applied in thin layers.
Old Masters paintings were mostly brown because most available pigments were brown. Earth colors are made from natural iron oxides that are found all over the earth in various shades of brown and muted shades of red, orange, yellow and green. Other minerals deposited with iron oxides, such as calcium, manganese, or carbon, effect their color. Impressionism would have been impossible without a full spectrum of pigments, packaged for the first time as oil colors in tubes. These are mineral colors and they were developed for every hue on the color wheel.