Signs for drivers

As I understand it, blue signs are generally informational, and red signs are generally warnings / instructions.

Oh hang on - here is a good resource: http://www.onlinedrivingtheory.co.uk/Signs/Introduction.aspx

From that it seems like the only signs that are to be obeyed are the circles/triangles eg blue circle for cycle paths and red circle for no cycling...

But bus lane signs are rectangles...confused.