What does the color of the LED mean in Detroit: Become Human?

This is indicator for Androids health and emotions for deviants.

Emotions:

As an indicator of emotion in deviant androids, blue is indicative of calm or contented feelings, yellow indicates apprehension and caution, and red indicates abnormal intensity in emotions, including anger, sorrow, and deception.

Health:

some functioning androids devoid of emotions in the junkyard have LED lighting in red and are missing several major biocomponents.

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Androids suffering from an extreme shortage of power may have their LED showing incomplete red ring, which shrinks until it is shut down.

Source - Detroit Wiki

It lights up in blue, yellow and red according to the amount, strain, and quality of an android's mental processing, physical condition, and emotions (in deviants). It will always indicate the worse condition of both mental and physical aspects. It also functions as an identifier of androidhood.

The color changes immediately according to their conditions, although it is rarely seen changing gradually along the LED ring.

In case the LED is flickering that indicates rapid or unbalanced activity, while a constant LED indicates a stable level of activity.

You can read more about it here.