text: The most common anodizing processes, for example, sulphuric acid on aluminium, produce a porous surface which can accept dyes easily. The number of dye colours is almost endless; however, the colours produced tend to vary according to the base alloy. The most common colours in the industry, due to them being relatively cheap, are yellow, green, blue, black, orange, purple and red. Though some may prefer lighter colours, in practice they may be difficult to produce on certain alloys such as high-silicon casting grades and 2000-series aluminium-copper alloys. Another concern is the "lightfastness" of organic dyestuffs—some colours (reds and blues) are particularly prone to fading. Black dyes and gold produced by inorganic means (ferric ammonium oxalate) are more lightfast. Dyed anodizing is usually sealed to reduce or eliminate dye bleed out. White color cannot be applied due to the larger molecule size than the pore size of the oxide layer.   
  Alternatively, metal (usually tin) can be electrolytically deposited in the pores of the anodic coating to provide more lightfast colours. Metal dye colors range from pale champagne to black. Bronze shades are commonly used for architectural metals. Alternatively, the colour may be produced integral to the film. This is done during the anodizing process using organic acids mixed with the sulfuric electrolyte and a pulsed current.  
  Splash effects are created by dying the unsealed porous surface in lighter colours and then splashing darker colour dyes onto the surface. Aqueous and solvent-based dye mixtures may also be alternately applied since the coloured dyes will resist each other and leave spotted effects.
  Another interesting coloring method is anodizing interference coloring. The thin oil film resting on the waters surface displays a rainbow hue due to the interference between light reflected from the water-oil interface and the oil films surface. Because the oil films thickness isnt regulated, the resulting rainbow color appears random.
  In the anodizing coloring of aluminum, desired colors are achieved by depositing a controllably thick metal layer (typically tin) at the base of the porous structure. This involves reflections on the aluminum substrate and the upper metal surface. The color resulting from interference shifts from blue, green, and yellow to red as the deposited metal layer thickens. Beyond a specific thickness, the optical interference vanishes, and the color turns bronze. Interference-colored anodized aluminum parts exhibit a distinctive quality: their color varies when viewed from different angles. The interference coloring involves a 3-step process: sulfuric acid anodizing, electrochemical modification of the anodic pore, and metal (tin) deposition.