Re: correct in camera colour
Rachel the filter is not left on the front of the lens. Just as you have presets ie AWB- shade- sun- flash etc this adds another varient in the preset section of DSLR cameras.
How it works is any photo as you know is worked out in grey and 18% grey is the correct amount, all colours a camera are based on this 18%.
Unfortunately most cameras are set well outside this amount therefore not do not produce the correct colour.
With the expodisc I had what it was calibrated to which was 17.8% which nigh on the 18% needed
So you set a camera up as normal pointing to the light source and take a photo. Then do the same in a preset mode with the disc clipped to the lens and adjust the setting in the viewfinder and take a shot. That results in a grey picture if seen on the screen. That is the new camera setting like the others so as long as that preset is left you can then take a photo as normal as that preset 18% grey is the basic grey the camera works to. if that makes any sense.
plenty of videos here on how what and why
https://www.youtube.com/results?search_query=expodisc
Don't get confused between colour and light which is easy to do, you have to think of them as completely different and unrelated
camera ones like AWB-sun- shade -flash etc are light based
Expodisc is colour based