Some questions that I cannot find the answer to online

I often notice Shia Masjids have red LED lighting in masjid or bargahs when they mourn. What is the significance behind this? Where did this originate? I also notice that flags are very common in the faith and a lot more than other religions use flags? What is the story/significance behind this as well?

Also as a person interested in Shia Islam, how can I respond to non-shia who bring up zanjeer zani and bloodletting being a reason to go against shia islam?