Blue For Bypass, Purple For Partly Bypassed
The answer is that some plug-ins can be be both bypassed and active at the same time. This is represented by the colour purple. This can be for two reasons:
Multi-Stage Plug-in with an inactive section
Plug-ins like Avid Channel Strip have multiple sections. Channel Strip has four: EQ, Filter, Dynamics and Volume. If the EQ or Dynamics sections are inactive but other sections are active the plug-in shows as purple.
Multi-mono plug-ins with an inactive channel
Any multi-mono plug-in can have its channels unlinked and if any of the channels is bypassed but other channels are not this partially bypassed condition is indicated by the purple colour.
Watch the free video to see this in action.