Light sources can have a color & intensity.
The default is full white.
blue = [0.1, 0.4, 1, 1]
glLightfv(GL_LIGHT0, GL_DIFFUSE, blue)
The color of a light defines how much red, green, and blue light is shining
on an object.
The color of a material defines how much red, green, and blue light it reflects.
The final color we see is the combination of the two - corresponding components are multiplied together.