This post is all about white balance and gelling. We gel (plastic color sheets) our flashes to make the color of the light match that of the background light. Otherwise if we balance for the flash, the background will either look yellow, green, or even worse.
When I first get to a room (at least 5 minutes early) I set my WB to daylight and I take a test shot with flash just outside the room. The door frame acts as divider preventing my flash from spilling into the room I'll be shooting (the back room). As you can see, my flash is much less orange-er than the back room.
I thought adding 1/2 cut of CTS (been using more than CTO lately) would be enough.....I was wrong. It's close, but still not orange enough.
So, I remove the 1/2 CTS and add a Full Cut of CTS (the only difference is the full cut is darker than the 1/2 cut - technically I could add two 1/2 cuts...but who wants to do math while taking pictures!). I take another picture and this looks much better and balanced. Now the flash looks like it's creating the same color as the room.
I also carry around a 14" foldable calibration target. Does a grey card work? yes, but I've had good experience with this, it was a gift, and has a built in reflector on the back that comes in handy. I take a picture filling the circle in a Canon viewfinder with the grey center and take an image.
Then I go into the camera menu, assign that image to the custom white balance, and then change my white balance setting to custom (check your manual)....voila! Here's the outcome:
It's always a good idea to go around getting your custom white balance images for the various locations in a shoot before hand and leaving the images on the front of the card. That way if you jump locations mid shoot, you can scroll immediately to the front by going past your last image taken, re-set the custom balance to your location and continue shooting.
Keep in mind, when there are different colored lights it won't be perfect. The room I shot in after this had Tungsten wall sconces, green overhead fluorescents, and daylight puring through windows all around.....just do the best you can!