The perception of color is based upon what photons of light are received by the eye. There are two types of dark matter: cold material that gives off little radiation of its own; and new types of matter whose properties have not yet been determined. The Earth is an example of the first type of dark matter, It gives off some radiation in the form of heat but the rest is reflected light from the Sun. If the nearest star had Earth-like planets they would be invisible to us, or "dark". There may be dead planets and stars in space that would also be "dark"
The other matter would also be invisible to our eyes because it would not give off light. Light reacts strongly with the matter we know today so if it reacts with light we should have seen some visible evidence of it by now. How do we know it is out there? We have studied the motions of stars in our galaxy. From the motion studies we can estimate how much mass is holding those stars in the galaxy. We find that from the motion there is more mass than can be seeing emitting light. We see further examples in other galaxies of more matter than can be seen. This new form of matter must not interact strongly with matter except for gravity. This suggests that there must be alot of it out there because gravity is essentially a weak force unless there is a lot of matter around,