Fill a shallow dish with water. Put a mirror facing the morning or afternoon sun. I tried this several times before we got it right. The trick apparently is to have as much water as possible and to submerge most of the mirror.
When I got tired of going out, I moved the mirror so we can have the window inside our house.
You can even project the rainbow on yourself.
Maybe if we have an aquarium and a big mirror, we can fill the living room ceiling ROYGBIV colors!