so, I'm shooting with the 6D and MkIII side by side on the same subject with the same settings. 4000ISO 2600k White balance f/2.8 1/50 of a second. the 6D skews a bit yellow and the mkIII skews a bit red. Should I attempt a WB shift to move them a little closer? or is this expected. btw, I have a 17-35 2.8 L on the 6D and the Tamron 24-70 2.8 VC on the mkIII. It doesn't seem to matter which lens. I tried the opposite and I tried the 50mm 1.4 on the mkIII it just never seems to match. Thoughts?
Bill

I think this is just a problem you will have to face. The only way to completely match is in post, even then it will be 99% which will be good enough I guess. Alternatively, buy another MK3.

You could make a custom color profile for each camera with the Canon software that is included on the CD. Then you would have a very close match from the start. Just shoot a color chart and start tweaking.

I have had the best time matching between cameras when using the Cinestyle preset. I'm not a total fan of this preset due to the low chroma recorded but it does make it really simple to match things up in post.

Have you tried using a custom WB instead of dialing in the Kelvin temp? I would suspect that both cameras will have the best chance at dropping a color cast if you WB to a gray card...
Pretty much always have to do at least a little bit of matching in post though, no way around that.