Refer to the scenario. A scientist sets up an experiment to see how colored lights affect the height of plant growth. He grows one group of plants in full sunlight, one group under red lights, one group under blue lights, and one group under green lights. All the plants are exactly that same type and all receive an equal intensity (brightness) of light. At the end of the experiment he measures all the plants. What is the independent variable?

B/c by definition of independent variables, these are the variables that are manipulated to see how it affects the other value (which is aka dependent variable). Color of light is independent because you have multiple manipulation which can be clearly seen in the different types of lights and the height is dependent on what color of light it is exposed to.

Additionally,

Dependent variable - the height of the plant

An independent variable is not affected during the experiment. It is what experimenter controls. The dependent variable is called dependent because it depends on the independent variables. Here, the height of the plant depends on the color of the light, therefore the height of the plant is dependent variable and the color of the light is the independent variable.