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On a cold night or if it's been snowing, jets are sprayed with orange de-icing fluid:

But why is it orange?

If it's to see where's been sprayed and where you've missed a bit, black would be the most contrasting colour on white; while yellowy-green is the strongest/most visible colour to the human eye; and blue is the least affected colour for colour-blind people.

Just curious as to why orange.

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    $\begingroup$ Here is a white one: imgur.com/kB05aNI $\endgroup$ – Antzi Oct 29 '14 at 9:47
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    $\begingroup$ Given the answer, this is no way off-topic. Who keeps flagging questions with no criteria? $\endgroup$ – Federico Apr 26 '16 at 18:09
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There are actually two types of de-icing fluid, one is orange and one is green. According to a product safety sheet for one brand:

The dye is intended to indicate which parts of the aircraft have been treated and to differentiate between fluids (Type I fluids are orange, Type IV fluids are green).

As to why orange and green, presumably they aren't easily confused with other liquids used around aircraft. Black would look like oil or another fluid if it pooled on tarmac, hydraulic fluids or oils are often yellow/green in colour and blue is a very common colour in aircraft painted liveries. So I guess that orange and green were sort of the ones 'left over' after all the other obvious colours had been considered.

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    $\begingroup$ Here I found there are 4 types: boeing.com/commercial/aeromagazine/articles/2010_q4/2 $\endgroup$ – Horst Walter Oct 28 '14 at 19:03
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    $\begingroup$ @HorstWalter: Yes. Notice that the two types in the answer are numbered one and four. The two in between seem to be obsolete. $\endgroup$ – Jan Hudec Oct 28 '14 at 19:05
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    $\begingroup$ Supposedly Type IV is favored over Type II now, and Type III is for slower aircraft. $\endgroup$ – fooot Oct 28 '14 at 19:07
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About your question on why black is not used even though it provides the highest contrast to white: thats not practically true. The problem is, that black and white have the same hue and color saturation (or rather they have no color saturation, and thus the hue is undefined in both cases), so they only differ in brightness/luminance. And a difference in brightness can easily be caused by different lighting conditions.

So for black-dyed de-icing fluid, you couldn't reliably tell whether a black-looking spot has been de-iced, or if it is just not well lit. Similarly, a white-looking spot may not have been de-iced, or may just accidentally reflect more light that its surroundings (e.g. due to specular highlights, retro reflectors, different incidence angle to the light source, or different surface reflectance), Colored fluids solve that problem, as color (e.g. green dyed liquid vs. white snow) barely changes under varying lighting conditions.

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    $\begingroup$ As an additional note, black does not contrast strongly with blue, and under low-light conditions, does not contrast strongly with red. White is the dominant color of airplanes, but hardly the only one. Orange, on the other hand, is rare. $\endgroup$ – Mark Oct 28 '14 at 21:48
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    $\begingroup$ lol - so Easyjet aircraft cannot be safely de-iced? $\endgroup$ – Martin James Nov 1 '14 at 10:52

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