ELI5 Fire Extinguishers - why are they red in color?

Why is most fire equipment red in color? What happens if you use the wrong class of extinguisher on a grease or metal fire? Could the future of firefighting be a "dubstep cannon" that uses sound waves to kill flames?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: jasonthefuzz, jebediah_johnson, rtpkickballer, audiblenod, markhunt1980, mpls_is_rivendell, dvn_rvthernot and dignitty.

To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter:

https://www.twitter.com/eli5ThePodcast/

or send us an e-mail:

ELI5ThePodcast@gmail.com