Grey Card, if you are in doubt about what colour the light source is, I recommend creating a custom white balance by taking a sample shot.
For those people new to photography from time to time you will create great pictures, but for some reason the colours look, funky.
Without going into the detail this can easily be corrected using a grey card.
Assuming your camera has the custom white balance function, all you need do is create a file of the grey or white card. Next step is to set up the custom white balance using this file.
The camera will look at the reference file and removing any colour cast from images using this profile when you camera has been set to this custom white balance.
Creating the sample file and setting up the custom white balance profile takes in total less than 30 seconds.
One last note, this profile will work only in the light your made the file in, if the light source or environment changes you will need to make a new file
There is another option, to sample from a neutral object in post production if you have this in your picture, that choice is upto you.
Professionals leave nothing to chance, I prefer to have a good starting point every time.