Why do colors change when you use more than one filter?

Question

My friend did an experiment where she use blue and yellow filters to look at a red binder. The binder turned brown. I know it has something to do with the combination of the three colors but why exactly did that happen?

Answer 1

The light coming off the red book is not a single or "pure" red, but a mixture of many different hues of red. The blue filter prevents those red hues that have no blue component from reaching your eye and the yellow filter prevents those red hues that have no yellow component from reaching your eye. If you use them together you'll only see those red hues that have both a blue and a yellow component - that's why you see brown.