L14-30 Plug & Generator
Use the right colors
First off, if you're wiring to US standards, you've got the wire colors wrong.
Green and bare copper are both used to identify an equipment grounding conductor.  White is typically used as a grounded (neutral) conductor (grey is also acceptable), though can be used as ungrounded (hot) if there are appropriate markings at each termination.  From there, basically anything that's not green, bare, white, or gray, can be used as an ungrounded (hot) conductor.
Wiring L14-30 Plug
To properly wire a NEMA L14-30 device, you'll need 4 conductors.

Proper Wiring
In your situation, I'd pull an extra red or black conductor from the panel to the receptacle location and remove either the green or bare conductor. Then you'll connect the original black conductor from the breaker, to X on the receptacle. Connect the new red/black conductor from the other terminal of a double pole 30 ampere breaker, to Y on the receptacle.  Next connect the white wire from the neutral bus bar in the panel, to the N terminal of the receptacle. Finally, connect the green/bare conductor from the grounding bus bar, to the G terminal on the receptacle.