Should is a form of Shall. Shall is related to Must. Must is related to Sign. 
Blue is a different concept from Red. Red is related to Primary. Primary is related to Lesson. 
Should --FormOf--> Shall <--RelatedTo--> Must <--RelatedTo--> Sign 
Blue <--DistinctFrom--> Red <--RelatedTo--> Primary <--RelatedTo--> Lesson 