Each train should have a number from origin to destination. Yellow 112. Orange 55. Blue 20. It accomplishes three important goals: it demonstrates to riders just how many trains operate every day, it implements a new measure of safety, and it gives passengers some semblance of routine.
It's a matter of safety in one aspect, but also a matter of accountability and transparency in others. How would you describe the train you are on in an emergency? Or if you left something on board? Or to report an incident after unboarding? "I was on the Yellow Line train that arrived at L'Enfant Plaza about ... 5 ... 15?" Rarely do passengers know what train they were on. It also instills transparency. If a commuter typically takes the Orange 55 from Clarendon every day and it's running 10 minutes behind schedule, passengers immediately understand the depth of the delay. It also provides Metro an easy opportunity to show just how many trains it runs every day. There are many ways to fix Metro. This may be the cheapest and easiest to implement new safety and transparency measures.