...according to me!
When Babs went to that infamous costume party where she ended up rescuing Bruce Wayne from Killer Moth, I reckoned she was in a black and grey costume because she wanted to look like Batman. Once she started crime fightighting, she decided to adopt a totally different look to Batman's black and grey, so she wore the purple costume. As she matured as a super heroine, she needed to be more stealthy so she switched to wearing black. After she recovered from her run in with the Joker, an older and wiser Babs would wear an armoured costume for more protection.
Oh and a belated happy new year to everyone. I hope you like my cosplay so far.