According to the BBC:
” The victims' lips and nails had turned dark purple indicating a lack of oxygen, while their internal organs turned "blood red", which is another sign of cyanide poisoning, said Professor Kornkiat Vongpaisarnsin of the Department of Forensic Medicine at Chulalongkorn University. “
Cyanide found in blood of Bangkok hotel victims https://www.bbc.com/news/articles/c3g64ljwp2go