Yellow everywhere

I have heard, and I believe, that we see only what we want to see.  Today I had a first-hand experience of the same.  A friend had shared Thomas Leuthard's book on street photography on Google+.  I take pictures now and then, but I am no photography enthusiast.  Nevertheless I am reading the book very very slowly.

One advice from the book is to train your eyes to see things on streets.  Thomas advises you to pick one colour -- any colour -- and try to see that colour throughout a day.  I tried that today.  I randomly chose red.  Suddenly it was red everywhere all around me!  I could see so much red that I thought I should pick a different colour.  So I told myself that I would see yellow.  Now I started seeing yellow everywhere, as much as there was red.  It's amazing how I don't see any of these colours when I don't want to see.

This exercise gave me a new experience, but my mind said this was just another variant of two different ideas I already have.  I could rationalise and see this experience as a proof to those ideas.  But I suspect that it's just my mind playing games with me and making me see only what I already know.  Maybe knowledge is, like everything else, both an asset and a liability.