Thank you, pdurrant! That was so great of you to write that out and make it so understandable (well, at least with the last part you wrote, I finally understood.
I didn't realize that this part made a difference in reasoning..
What the governor's speech has done has been to convert the fact that there are both colours in the prison from a fact known to all the prisoners to a Common Knowledge fact.
I also got lost when I was trying to figure out how many saw what and thought what on the succeeding days, by using the 100 total. I should have tried it with smaller numbers first.
Many, many thanks for the explanation and for the fun puzzles.