Are you actually going to Colon or Shelter Bay Marina?
Colon is a slum BTW, be very cautious about going into Colon proper.
Taxi. About a 1 hour drive. About $100 fare.
Public Buses. Cheap
, but means disembarking in the Colon bus terminal and getting a taxi from there...dont even think about walking anywhere from the bus terminal.
Train. Cool option if it fits your schedule. Leaves Panama
City around 0715, one hour trip, about $25 IIRC.
Shelter Bay runs a weekly shuttle into Panama City. If you are meeting a boat at Shelter Bay you might be able to ride back on the shuttle.
Line Handler. Hitch a ride on a boat transiting as a line handler. Schedule is of course not very predictable.