Numbers on the climbing wall

An original explanation of an invented indoor route display.

The wall had twelve numbered routes. The numbers identified the routes; they did not rank them from easiest to hardest. A separate colour on each label indicated the level of challenge. On the first morning, several visitors chose route one because they assumed it would be the simplest.

We added a key beside the entrance and removed the word Level from above the route numbers. The key showed the meaning of each colour, while the numbers still helped visitors tell staff which route they meant. We had been using two different systems of information on the same small label. Once their purposes were clear, the labels became much easier to use.

What did the colour on a route label indicate?