Guide to the effect of impurities in Al

Impurity Affect
Iron effects brightness at 0.08%, but at 0.3% causes problems from a uniform light grey, striated or mottled appearance according to the distribution of the iron particles.
Silicon leads to cloudiness when present in alloys in excess of 0.8%
Titanium effects brightness.
Manganese can cause brown, grey, or mottled finishes at 0.3% on thicker anodic films.
Chromium at 0.3% leads to coatings which are yellow in colour.
Copper, Zinc & Magnesium are present in the above alloys listed but are in % concentrations that should not cause problems.