The 18% grey card is just a reference to the timer or colorist as to what "normal" is, a neutral frame of reference. So it should be lit flatly in as boring a lighting set-up as possible so it can't be creatively interpreted (for example, if outdoors, shoot in front light, not back light).
The exposure should be normal for however you are rating the film stock.
And the color of the light on the card should be whatever you want to be timed as "white" light. If you shoot the card in orange light, then the timer will add blue to the image to make it look like a neutral grey card instead of an orange card -- thus causing whatever follows the card to look bluer in comparison.
So normally you would shoot the card in "white" light (under a 3200K light indoors on 3200K stock, or in daylight with an 85B filter on the camera when shooting on tungsten stock, or no 85B filter if using daylight stock, etc.) although sometimes you may use non-white light or a camera filter to "trick" the colorist into shifting the color of the image in the opposite direction. The most common variation is to shoot a grey card under the greenish light of flourescents so that the timer adds magenta to correct the light to look "white". But another trick would be to add a light blue gel to the light on the grey card so that by timing the blue out, the dailies look warmish.
