In an interesting twist, when Apple made icons a little more complex with the addition of Retina icons in OS X 10.7, they actually made the process of creating the icons a lot easier all you need is a little skill with your favorite graphics program that can create images with transparent backgrounds (such as TIFFs or PNGs), and you can crank out custom icons for custom folders, internal and external drives, or even applications using built-in utilities and features of Lion and later.
Some worked well, and others not-so-well. Later on, I wound up registering a Developer account with Apple so I could use the icon tools in Xcode (well… and for AppleScript Studio, but that’s a different story), and probably tried a half-dozen different freeware or shareware utilities over the years.
Back then, I was stuck with 256 (or fewer) colors, a black/white alpha channel, and pretty much had to make them pixel by pixel in ResEdit (uphill… both ways… barefoot… in the snow…). I’ve been making my own Mac icons for folders, applications, and a myriad of other uses for well over 20 years now.
