Or, another thing I always try to do is have a white balance reference card in at least one of my shots. I can then use that to sync my photos later when I transfer the images in my computer.
(Take note, however, that you should have a reference point for every lighting situations: if the lighting set up changes, then you will have to shoot your grey card again.)
I always have my X-Rite Color Checker Passport in my camera bag. This is a very good reference to have: you can have a reference for landscapes and portraits; you can choose to warm up or cool down your photos; you can even use it to calibrate your cameras.
But for those times that I don't have a camera bag, or occasionally when I forgot to transfer the Color Checker Passport when I change camera bags, I always have an emergency reference card in my wallet.
The Opteka Pocket-Sized Reference Color & White Balance Grey Card Set (yes, it's a mouthful) is something that I always have in my wallet. It is a relatively cheap set of white, grey and black cards that you can put in your wallet. And since I always have my wallet, thank goodness I haven't forgotten my wallet yet, I always have an emergency reference point whenever I forgot my Color Checker.
Sometimes I choose not to change the white balance of my photos, sometimes it doesn't even matter when I convert my photos to black-and-white. But when I want to be precise about the color of my photos, I always have a reference card to make my work very easy.
So get yours here: