You can do things with your handcuffs that will get you arrested
There are regulations in every department about what you can/cannot do with law enforcement equipment, especially that which is officially issued to you. Most officers have additional gear they've purchased themselves - for example, I have an issue pair of cuffs, but, since I prefer always to carry 2 sets, I had to purchase a second set myself. Some departments prohibit the use of certain types of equipment - some kinds of cuffs are easier to pick, some holsters are considered safer than others, and so on. A lot of us dislike the issue body armor and fork out for our own - an absolute necessity in my case, given my particular figure; I also had a heckuva time finding tactical boots until just recently, when it finally dawned on suppliers that there are a lot of women in law enforcement now and we don't all wear Bozo the Clown size shoes. Issue equipment is also "accountable" which means you have to keep track of it and make certain you turn in the exact item you were issued, or pay for replacements at government prices.
Within reasonable limits, at my agency we are permitted a fair amount of discretion with personally acquired equipment. Colored cuffs are a recent development and I understand are very popular with agencies who do a fair amount of arresting and prisoner transport, as it makes it much easier to ensure you get your own cuffs back (when they all look alike, it's not unusual to get a crummy pair of lowest-bidder cuffs returned to you rather than your own expensive set). Most of us have a certain type of cuff key we prefer as well (i.e., on a keychain, pocket clip, etc).
I think TPTB also consider it a fairly harmless way to let the "kids" goof around and blow off steam. At the medical examiner's office I most often work with, the mid shift likes to decorate the eye protection with stickers, which means you have to be pretty quick if you've got a pair with unicorns on them and the recording camera pans over to you.