Cyanosis

This seems like a random curriculum item to me. A quick trawl through the Oxford Handbook of EM suggests I need to think about:
Anaphylaxis
Asthma
COPD
Aspiration
Pulmonary Embolism
Seizure
Toxicology - opioid poisoning, paraquat lung, petrol and paraffin poisoning, anthrax
Respiratory distress - chest wall injury, pneumothorax
Uncommon in cyanide poisoning

LITFL splits it further
Central Causes - low sats, metHb, altitude
Peripheral Causes - cold, reduced cardiac output including congenital heart disease.

References
http://www.rcemlearning.co.uk/exams/he-looks-a-bit-blue-doctor/ 
http://www.rcemlearning.co.uk/modules/papa-smurf-has-a-seizure/ 
http://lifeinthefastlane.com/resources/cyanosis-ddx/