Probably another silly question, but my white balance cards are being delivered today.

When I use them, do i have to do a test shot everytime I change power settings of my off camera flash OR just one test photo with lighting in place and firing? I know if natural light is changing I would have to fire another test shot - but not sure if the colour of light changes with different strength of flash firing.

Hopefully the white balance card will help me get images without the colour cast I seem to keep getting.