Bumper plates in most gyms follow the same colour code: red for 25 kg, blue for 20, yellow for 15, green for 10 and white for 5 (sample note).
We use the same colours on the timetable. A white class is for your first sessions, green is easy, yellow is steady, blue is hard and red is heavy.
Nobody has to stay on one colour. Most members mix a red class with a green one in the same week.