Rowling always describes spells as colors, usually with a descriptive word like "sickly" or "sluggish", particularly when describing spells that are dark/evil or that Harry and co would have no way of knowing. When it's something they're familiar with, they'll name the spell, like "stunner". It's a generally accepted fanon invention that the AK is a much more difficult spell to use, or at least takes a degree of power or other ability to use, primarily based on what fake-moody says in book 4 about the whole class combined not being able to do anything with it.

~DarkSov