The drought in north China is getting worse. officials changed the warning color from blue to yellow which alarms advancement in drought severity.

中国北方旱情日益严峻 官方已经把旱情预警级别从蓝色提升到了黄色。

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Other warning signs are a change in skin color and uneven borders around a mole.

VOA: special.2010.07.06

Authorities have also introduced a color-coded warning system to alert residents about dengue outbreaks in specific districts.

WSJ: Dengue Fever on the Rise in Singapore

New York has been on "orange" alert, or the second-highest level -- indicating a high risk of terror attack -- since the color-coded warning system was established after the September 11, 2001 attacks.

CNN: New York subway system on alert

One application Sotzing has been working on is meat packaging with a built-in tongue that can taste the first signs of spoilage and activate a color change as a warning.

FORBES: Safer And Guilt-Free Nano Foods