The role that they play is they break down dead plants and animals and waste(poop). This allows for plants to get nutrients in the means of fertilizer from what the decomposes break down. It also prevents from dead plants and animals piling up.
light is a form of energy expression, so knowing the law about energy conservation, we can conclude that the light absorbed by the dark object doesn't disappear but turned into heat, which is kept or emitted by the dark object.
So Yes,
the darker the color, the better the heat radiation is