I going to cover two things here.  Firstly, how to get the Canon C300 and the C300mkii to roughly match when filming, and secondly how to get them to match exactly in the grade.

There are several different options when it comes to matching the Canon C300 with the mkii.  Here are a few of them:

1. Use a REC709 profile.  The advantage of this is that, if there are other cameras on set, they will also be able to use a 709 profile as most cameras have it. (On the original C300 profile 3 is 709)
2. Use Ablecine's profile.  AbleCine have have made a number of scene files for many different cameras, Arri Amira, Sony Fs7, F5 etc.  
3. Shoot LOG.  Canon Log on the original C300 has a corresponding LOG on the mkii

The video below shows all three of these option and there is also a shot which shows the LOG footage with the added LUT.

As you will see in the video none of these work as a perfect solution in their own right, each will need tweaking in the grade.  This is hardly suprising, if it was easy to exactly match one camera to another,  a camera manufacturer would bring out a cheap camera and match the profile exactly to the Arri Alexa, and everyone would buy that.

Incidentally the oringal EOS profile would probably match up quite well between the 2 cameras (simply as it has the most acurate colour rendition).  However, I decided not to use this as I have always found the EOS pofile to be too heavily saturated, the reds also pop like crazy.  

In conclusion, I would favour the LOG profile here.  Although it doesn't match exactly, it looks reasonably close and with a small amount of grading could be improved upon.  Also, by shooting LOG you give the colourist or editor a bit more info to work with.

In my video I simply added the LOG to 709 profile, just to show how the cameras match up before grading.  However, with the One Shot chart you can create specific LUTS which will match each camera.

Grading systems like Resolve and Baselight can match the cameras by selecting the chart from the menu.  I am using the One Shot chart from DSC Labs, which Art Adams came up with.  You can buy these from DSC Labs direct, or from the usual places -B&H etc.   The video below is by Michael Garber and shows how Resolve matches cameras using the One Shot Chart.

 

To buy the chart from B&H use the link Front Box One Shot Plus ("AcuFlect" Glossy Finish)