I've just encountered what appears to be a small bug.
If I create a line view layered by a field and select the option to "include excluded values grouped as 'Other'", then the "Other" doesn't appear in the colour codes, and makes the view somewhat confusing. I'm using 2.7 b422
In 2.7, the othering uses a default data colour, if it contains mixed values for field you are colouring by.
We don't show colour keys when default colour is being used. You could change the colour to make it easier from Style > Data colour > Default data colour.
In 2.8, there is now a new option in the Style> Data colour menu called "Mixed data colour" which determines the colour to use for othering. This is similar to the "Default data colour". Just like "Default data colour" we don't show the colour in colour keys
One way to fix the problem you are experiencing is by including an option to include "Colour key" for othering.