Simulink uses red edges and signals to highlight blocks and signals in a few cases (there may be more):
- When there is an algebraic loop, the diagnostic can highlight the loop in red
- When displaying sample times, Simulink uses red (amongst other colors) to show blocks with the same sample time
I have experienced when I am using these features, Simulink highlights blocks and signals, but sometimes does not un-highlight them and I need to manually change back the edge and signal color to black. If you are getting no significant errors or warnings, then this may be the case.
