Scanned items should have a scale included right next to each one. Something like this would be perfect:

Any correction then would be performed to match the scan to the actual scale.

I hope your scans are like this one (Song Of The South production cel with Walt Disney signature):

BTW, if I take the present raw scan:

normalize the RGB spectrums (expand to 0-255):

and match only 2 face colors (mouth & cheek) to the above production cel:

we'd have a targeted color correction. :)