From the get go, I have absolutlely no idea about said subject, although
from my limited logic processing:

These Slides would be photographed in what format...????

Said format would have to be able to be converted to a wireframe format of
sorts thus placing each pixel in columns and rows so they are
seperated/isolated from each other so as to individually count pixel shade
variations, importable into an Excel spreadsheet.

Lets assume for the sake of the exercise that it was possible:

Sheet1 has the before and sheet2 has the after, you could look at a Case
Select argument whereby you would have something like

Case Select True

=CountIf(MyRange.BackColor: = Brown)
=CountIf(MyRange.BackColor: = Black)

Etc

Just to give a hazy insight to what maybe plausible given the above
conditions....

The exercise of comparing between before and after slides will be somewhat
moot if you can't at least go some way to seperate each pixel shade. (using
excel as a platform at least)

Having said that, once each Sheet is compared, you will have the difference,
then again, I may be totally wrong and get shot down in flames by someone
who has clearer insight as there are several Demi-god MVP Guru's who may
have a better analagy and understanding who can bring alternatives to the
table.


Good luck

HTH
Mick.