On a general house line clean if you use a purple line clean solution should it turn dark green on mild and bitter?
You should look to the instructions of use that came with the container. There are a lot of different types of purple line cleaner out there. Some change colour indicating yeast is still present in the lines while others, are purple just to indicate that cleaning solution is present in the line. To answer your question the mild and bitters generate more yeast than lagers and sometimes can produce some pretty colours. If it is a yeast indicating purple line cleaner, pull fresh solution through every ten minutes, when it stops changing colour this indicates that the lines are supposed to be clean.