Missions come in different types, distinguished by colors.
Key Missions: Main Story Chapters, Special Episodes, etc.
Agent Missions: Agent Stories, Invitation Missions, Quality Time, etc.
Hearsay Missions: Combat Missions, Exploration Missions, Schedule Missions, etc.
Event Missions: Limited-Time Events, Ridu Chronicles, etc.

You can distinguish the current mission status by the shape of mission icons. Taking Side Missions as an example:
Available to accept;
Accepted but not tracked;
Accepted and currently tracking.