he at every opportunity made note that I had a mid grade canon (5dmkiii)
First, the 5DMkIII is not a "mid-grade" camera. It is a fabulous camera, one that professionals could only dream about just a few years ago. Lots of pros use it. In fact, at least three of the wedding photographers I have seen recently were using Canon 5-series cameras.
The comment about a color cast is nonsense.
First of all, if you want to get the best out of the camera, you should shoot raw, which makes most of this discussion moot. The major issue of color casts comes from the choice of "picture styles," which by design process the photo to have a certain color mix, saturation level, etc. Take the same shot with the standard and landscape styles and you will see this. Those styles are irrelevant if you shoot raw, because a raw file contains all of the original data. The software may read the style for an initial rendering, but that has no real effect.
With any camera, if you are concerned about maximizing color accuracy, you can use a color checker and set a profile for the camera. I have been using Canon digital bodies for 6 or 7 years (currently, a 5DIII) and have never seen a need to do this. I use a white balance card if I am uncertain about lighting (you have to do that with any camera) and use the Adobe Standard profile for an initial rendering. I adjust WB in post, but I only rarely have to tweak individual colors. I can't even recall encountering a magenta cast.
The real bottom line, to paraphrase Ansel Adams, is that the most important piece of photographic equipment is the 12 inches behind the viewfinder.