An anodizing layer may be dyed or undyed. In the 8625 Mil spec, the term “Class” is used to designate this: Class 1 is clear or undyed, and Class 2 is dyed.
Anodizing layers are commonly dyed with organic dyes by dipping the part into a hot dye tank directly after anodizing. A wide variety of colors can be obtained through this process. By far the most commonly specified color is black, followed by blues, reds and golds. Note that only Type II anodizing can be dyed a variety of colors. Type III “hard” anodizing, which is quite dark even when undyed, is typically left clear or dyed black.
Colors and Color Matching
It is important to note that colors obtained through dying are not as consistent as those obtained through painting or powdercoat. There is no such thing as a pantone matched color in most commercial anodizing, and consistency in color and color matching is extremely difficult. Generally colors are specified in a broader way than with painting, e.g., you would specify “blue” or perhaps “light blue” or “dark blue” for an anodized part, not “deep sky blue” or “electric indigo blue”.
Note that colors obtained through organic dyes are not absolutely colorfast, especially in high UV exposure situations. All organically dyed parts will fade to some degree over time. Outdoors, the fading can be dramatic.