Why are all colors not visible at all times or visible some times?
Colors are what we we call the different perceptions our eyes have of light reflection or absorption. An object's color is determined by physical properties such as rate of light absorption, reflection, and emission spectra. Our perceptions of color can differ from person to person depending on how sensitive the cones in your retina are and how much they are stimulated. Cone cells have three light types it can detect - red, green, and blue. How these frequencies combine or how your individual cones interpret them can explain why you think something is turquoise but someone else only sees blue or green.
Color perception depends on what light wavelengths are reflected back from the object. Take for instance an apple. The physical properties of an apple are such that it absorbs every color BUT red. When all light wavelengths are reflected, then the color we see is white. In a way, you can think of white as being all colors at the same time. Black is the color we see when no wavelengths are absorbed (so, in a way black is no color at all).
The reason why we can't see color in the dark is because we can't distinguish enough light to see the color reflected back.