Prior to learning to use the settings on my cameras to get out of the default linear curve, everything I have shot for years was based on the default settings of my cameras. Is there anything that I can do to my old videos to make it possible to color grade it? Is everything baked in with not much room for doing anything at this point after it is shot? Some of my cameras include the A7S2, the RX100V, the FDR-AX100, the 5Dm3, iPhones.
it all pretty much matters on one thing, the dynamic range of the light and the sensor. if you shot soft light and properly exposed and no clipping then they will all be theoretically able to match each other. You will have to grade to the lowest quality denominator though. probably the iphone as it only has 10 stops dynamic range.
Whenever I have completely different footage, I first neutralize everything I have to 7ire blacks and 90ire whites so that I can add contrast later. It keeps my eyes from being blinded by 100 ire stuff and I can also view unbalanced blacks in my RGB waveform. You will probably also need curves with 5 points per color channel. blacks, shadows, midtones, highlights, and white point as the sensors respond differently around the luma curve. This is called '3D white pointing'
as a regular white balance won't correct the tones separately, especially if you have strange light sources.
Depending on the sensor quality, you may also need to rotate some colors' hue points so the skin tone line has the same red/yellow response.(that's usually in low light problem stuff)
There's going to be a lot of waveform comparing 'shapes' and eyeballing random unconnected shots. If you can mask the shadows, midtones and highlights on the hsl vectorscope, then you will be able to neutralize any weird color casts as you won't be fumbling on a curve adjustment eyeballing one shot forever trying to get it just right.
Anyway, others may have a different workflow like directly match one camera to another. I just find neutralizing to pure waveform, and then matching up kind of untangles the web of problems by itself. You may come up with a better way.