Cool,
On mine i have just ran all the led's directly off the arduino, i.e. each led, i connect the anode through a suitable resistor to run it at 15Ma on 5 volts, and then to the arduino's output pin i want to control that led,
All the cathodes from the led's are tied together and go back to one of the GND pins on the arduino (the one next to pin 13 on the uno is connected to the ones by the 3.3 and 5v pins on the other side of the board btw)
Now, i may be stressing my arduino by doing this, i believe the max current that should be drawn from any 1 pin is 20Ma.... and i have just realised i am running about 10 white led's off one pin for the instrument backlights.. oops,
I am not sure if the current limit applies when the arduino pins are sinking the current... someone else can answer that as i am just not sure,
But for now, i'd not bother with an external power supply on your set up, as it's only 6 leds, and the arduino is more than capable of running that load,
i only need 12 volts in my set up as i have automotive gauges that run on 12 volts, i also have 24 volts for a few illuminated buttons that just do not look right with led's in them, but as i mentioned, when mixing voltages, you really need to keep them seperate and away from the arduino,