Below is a basic chart on how various metals react with aluminum in anodizing.
This is accurate based upon our results.
Iron Reduces the brightness even in small amounts. Its addition to pure aluminum leads to dark grey or black streaks, especially when high iron to silicon ratio
Silicon At 5% level alloys, dark grey or black tones are obtained.
Copper Up to 2% can give alloys with a clear protective coating. At higher level, anodizing becomes more difficult and the coating is softer and with less protective value
Chromium At 0.3% leads to coatings which are yellow in color.
Titanium Reduces the brightness, as iron, but is added to produce finer grains
Magnesium Can give clear colorless coatings up to 3%, because magnesium oxide has a reflective index (1.736) very near to that of aluminum (1.69)
Manganese Up to 1% may anodize to clear silver, grey brown or mottled according to production process and particles size of the constituents. With thicker anodic films, manganese contents of 0.3-0-0.5% give quite brown anodic oxide coatings on all the alloys.