Flumes, like summons or attributes, are classified by their Grade but have a secondary means of identification.
For starters, as with what happened when Aoni first upgraded his Flume to the full-star Step, it will glisten as if covered with stars, a testament to its mysterious nature, but beyond that is their color.
For example, for the Young Master's Emerald Flume, by merely seeing it for only a second, Aoni was able to determine not only its Grade but stage as well, solely based on its physical color, Emerald.
Mind you; he could have very easily simplified it by saying, 'Hey, Green Rock,' but that wouldn't necessarily describe its Step, only its Rank.
To understand the color system, one needs only look at the color wheel.
Red, orange, yellow, green, blue, and purple are the most known colors flumes can reach, but as explained, that's merely a generalization because within each "Stage," the steps are based on gems.
