The independent variable is the variable that changes and affects the dependent variable. In this case, the independent variable is the COLOR of the FLOWER. The changing variable is the color because you want to see if the hummingbird will go to the yellow or red ones.
The humming bird in this case is the dependent variable because it cannot be change and it is the one that is affected by the changes of the independent variable.
Again, the answer is the color of the flower.
