Color is the way our eyes determine the wavelength of light. We can see light between about 350 and 750 nanometers of wavelength. The low end of that we see as red, the high end as violet. So, the different colors are different labels we put on visible light.
Religions are a set of belief structures, almost always accompanied by rituals. They describe different ideas about the supernatural. They typically, although not always, posit some god, or supernatural beings. They too have a wide, multidimensional spectrum of possibilities: monotheism, polytheism, animism, etc.
Darkness is the absence of color. If someone says “what is the color of darkness”, the answer is that there is none. You could say “black”, but “black” is not on the color spectrum, it is simply a word we use for the absence of color. If you are shut in a perfectly black box then there is no light reaching your eye and therefore there is no color.
Atheism is the absence of religion. By definition, it is not another religion, it is the absence of religious belief. There is no dogma, no belief structure. If two religious Mormons meet then they have a pretty good idea of beliefs that they share (although they may differ on the details) and of course, the same applies if two Hindus or Jews or Muslims… etc. get together.