Of course nothing is simple.
Please expand your explanation of your goal. Pulsing an LED on for µS is a lot different than operating for seconds.
- Thermal concerns
- Transient response of the current source
If my guess is correct you wish to use these LED as sort of a color filtered flash then you are best using a simple resistor and Mosfet for on off. You will need a Mosfet driver as the Arduino cannot supply enough current at its outputs to switch a Mosfet very fast.