12v RGB Leds - Using GPIO on Duex

[PaulHew]

I was following this link on RGB lights.
DC42 mentions that the GPIO ports can be used.

My RGB LEDs have a common positive and then neutrals for the colours.
I have changed the V_Fan select jumper to 12V and tested using a fan output, all works.

My question is
Can I connect the negative wires for the individual colours to the GPIO pins OR do I need something between the pin and the LED strip, and will I be able to dim each of the channels on the GPIO outputs?

Thanks in Advance,

Regards,
Paul.