He literally turned pink in colour.
I haven't read the book, but it's clear from the context that the subject had been inhaling automotive fumes. This causes carbon monoxide (CO) poisoning.
The CO binds to the haemoglobin in the blood preventing oxygen being released from the blood to the tissues of the body. The tissues and organs are starved of oxygen, while the oxygen in the blood goes up and up. The haemoglobin in the blood turns pink when bound with oxygen and CO, and in some patients having all that pink blood can lead to them having a flushed or blushing look to their skin. This has been a favourite clue in murder mysteries and such like for a long time.
The pink colour is actually visible in only a tiny number of CO victims and so the fact that someone hasn't turned pink doesn't mean they're poison free.