I almost made the picture editor use 0-100 as well, which I do think makes more sense, but in the end I went with what other image editors do. Everywhere else I try to use a percentage where possible, such as with opacity which I feel should definitely be 0-100 and not 0-255.
The reason everyone's used to things going 0-255 is simply because it's common to store pixel data in an 8-bit unsigned byte, which has that range. Early graphics software simply displayed these values directly to users, and I guess it caught on.
The thing is an 8-bit byte is just one of many ways to represent color values, and it's possible to have 1-bit, 4-bit, 5-bit, 16-bit, 32-bit and even float 16-bit and 32-bit variations, or mixed-bit integer representations like a 16-bit color storing 5, 6 and 5 bits for red, green and blue respectively. In fact many modern graphics processors process everything as floats from 0 to 1 in a variety of bit widths and multiply by 255 just before displaying them on the screen.
In short 0-255 is not "the" way of entering color values, and may in fact be totally unrelated to the real representation inside the computer, so it's actually kind of an arbitrary choice these days. It's just everyone's used to it and probably assumes colors are still 8-bit values even if they aren't so much any more. So IMO it makes more sense to make it 0-100, like many GPUs really use but multiplied by 100 for convenience, but I went with convention for the image editor because it seems to be a de-facto standard there.