Oxide finishes are commonly used to colour architectural precast concrete. Oxide suppliers publish brochures of standard colours. Colour classification can broadly be described as Black, White, Yellow, Brown, Red, Green and Blue, all of these colours can be blended to produce customised colours. The first four colours are the most economically available and widely used, where as Green and Blue are extremely expensive and as a result rarely ever specified. Oxides are commercially available in either powder or liquid form and are generally added into the mix at the concrete batching plant.