the whole front of the car including the front bonnet lock brace is supposed to be painted black - that way when everything is mounted, lights, grille etc. you wont see white paint peeking from behind the louvres in the bumper and grille.
Get some ABS plastic sheets or rubber sheets, the export models corolla had rubber/plastic air ducts that mounted on top of the grille to the radiator support, on the sides of the radiator to the front bumper and on the bottom edge of the bumper to the radiator/condenser. It ensured that when the car moved all cooling air hitting the front louvres actually got forced through the condenser/radiator.
Takes about 2 hours to make, a few finishing clips, screws, thin sheet metal and plastic/rubber sheet are required.
and yes CNG and the quad throttle 4AGE are not a good combination.