scientifically speaking rainbows can happen even though it hasn't been raining. a rainbow (scientifically ) is light passing through water molecules. there is always water molecules in the air (evidenced by clouds, fog, dew, water evaporation from oceans, seas, lakes, rivers, streams) I remember in high school we made a rainbow in a peanut butter jar just by screwing on the top and placing the covered jar over a flame and shining a flashlight through the glass jar. there is always moisture in the air and as long as there is a sun or other light source there will always be a rainbow, whether we see them or not just depends upon our position compared to where the light source is shining through the water molecules...

that said spiritually I believe anything is possible. it just depends on your own culture and religion as to whether you believe it was your pet communicating with you....we cant tell you whether it was your pet or not...

example one person on here from one religion (cant say directly as talk of specific religions is not allowed in this forum) it would be believed that yes it was your pet, in another religion it would be believed that no it was not your pet because the pet is waiting for you at the rainbow bridge and yet another religion teaches no that all things are born, live, die then are reincarnated to be born and live again completing what ever tasks that was left undone and righting any wrongs they have done in their past life....

so ultimately its not what others believe....its what you yourself believes. if your religion and culture says it was your pet then it was.