Fire drywall in bathroom
Do you mean purple rather than red drywall?  I see that you are from Spain, so I don't know if this answer will apply in your country - but here in the USA, the difference is that green drywall is moisture-resistant, but it is not waterproof.  Purple drywall has superior moisture- and mold-resistant characteristics that make it much more suitable for use where it might be in contact with water.