The yellow(straw) color is a particular form of iron oxide. Very thin colored oxides form on the steel surface as it is taken through specific temperature ranges, provided of course the metal has been polished first. Straw, light blue, a purple of sorts, and dark blue are all obtainable from heat treatment. The purple seems to fade to bluefairly quickly. The blues and yellow are fairly stable. The coating is fairly thin on the straw. Anything that will stabilize the oxidation process will help preserve it. I have no idea of the effect of sunlight or ultraviolet on it.
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