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I sauteed some fresh cut veggies in a stainless steel wok and it has a rainbow tinge on the inner bottom? Is it safe to continue using this wok for future cooking?

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If the pan has been always used in the kitchen and for cooking,

then is absolutely safe.

The phenomenon you observe is due to thin film interference. Is the same iridescence that we observe on soap bubbles or where a thin film of a greasy matter spread on a water surface.

In the pan case, the thin film causing the interference and the related coloured fringes is formed by metal oxides already present in the pan. Actually these oxides are exactly what makes the steel of the pan stainless and resistant to corrosion.

The phenomenon can be amplified if the pan has been overheated for instance while sauting to dryness.

The rainbow might be an annoyance but does not compromise your pan nor the quality of your food.

For those onto the physics and chemistry of this here are two possible starting links:

https://www.scientificamerican.com/article/why-doesnt-stainless-stee/

https://en.m.wikipedia.org/wiki/Thin-film_interference

In principle a thin layer of grease/oil can give the same effect. This is obviously not the case for a washed pan.

EDIT: As for OP mentioned a stainless steel pan I forgot to say that obviously also a non homogeneous thickness of reticulated fat can lead to fringes or spot in a seasoned iron pan.

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  • Could be polymerized oil (also presumably safe, given it is used as a seasoning in cast iron pans)?
    – rackandboneman
    Commented Jan 8, 2018 at 0:48
  • @rackandboneman I see what you mean. Like in old warnish based on oils. Honestly I am not sure. It might be a possibility that some reticulation happens giving the same effect. But I would see it as a transient phenomenon as the " warnish " should burn or continuously detached while cooking. Honestly as a chemist I cannot exclude something like that at least in principle. The rainbow should detach anyway during standard cleaning of the pan, as I would not expect any mechanical resistance from such as hypothetical but thin film.
    – Alchimista
    Commented Jan 8, 2018 at 11:56
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    @rackandboneman : polymerized oils on stainless steel are typically orange and splotchy (turning brown if you then overheat it). The rainbow effect comes from overheating a dry pan. (leaving it too long to pre-heat, when I've done it)
    – Joe
    Commented Jan 8, 2018 at 14:11
  • @rackandboneman indeed seasoning an iron pan means reticulate some oil or fat on it. I did trascure that as OP mentioned a stainless steel, but the same can happens.
    – Alchimista
    Commented Feb 12, 2019 at 10:22
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    I've discolored stainless by heating to a dull red glow. The pan is never the same, but you can cook in it. The color comes from having changed crystal structure and tempering, not any food substance. Don't do this.
    – Wayfaring Stranger
    Commented Feb 13, 2019 at 0:11

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