The Barcelona man was sent off for elbowing Croatia striker Mario Mandzukic in the back as Volker Finke's men slipped to a second defeat at the FIFA World Cup, going down 4-0 in Manuas.
Cameroon are now out of the competition and also saw Benoit Assou-Ekotto involved in an unsavoury tangle with team-mate Benjamin Moukandjo late on, an incident coach Volker Finke described as "unimaginable".
Song was compelled to say sorry following his first-half dismissal.
"I'm very sad at the moment as I feel I have let my country and myself down," he said in a statement.
"It was a stupid moment and I'm truly sorry. If I could do anything to take it back I would. Please forgive me."
Cameroon will seek to restore some pride when they face hosts Brazil in the final game of their campaign.
