^Agreed. The colors aren't glaringly bright, but they're there. Supposedly, there's a plan to make US currency much brighter, with variations in size and texture for each denomination, in the next redesign. The purpose is to make handling cash easier for the visually impaired. I'm guessing they didn't go whole hog with the colors this time around in order to ease the public into it.

I don't know if there is actually a reason for it all looking the same at one point, but if I had to guess, it might be that it was easier to just use the same type of ink for all the bills? Newer bills above $1 do have slight variations in color. The size of the president in the middle is also different on the new higher bills than on the single. It's not super obvious, but I remember the first time I saw each of the new bills, it was striking. There are still "old" bills left in circulation, though, because they didn't say "okay, we minted enough new bills to replace all of the old ones at once, come trade!", but they're slowly being removed. But really, the numbers are quite large on US currency, so if you aren't visually impaired, it shouldn't be that hard to glance at the bill and see what it is. I've also heard that they're going to make the bills more different. But again, it's not like they can instantly change out everyone's currency. And, once these new bills come out, they'll have to eventually change all of the bill reading machines- which will take a lot of time and money

Our system is just a little locked metal box with a hole in the top. The box can't tell if you put in too much, or not enough, and it can't give change. The driver can't tell whether you've put in way too much money, he just assumes you're smart enough to only put in the fare, and not another $198. Generally, if you don't have change or singles, you can ask other passengers to help you make change.